If I had a power of attorney drawn up after my mom’s diagnosis of dementia is it null and void?

You cannot have a POA drawn up. The person who wants to give you POA over their financial dealings is the one who has to do that, and they MUST be competent to do it.
I don't understand what you mean by "I received POA to help with her bills and care". POA is not given to you by anyone but a COMPETENT adult who is capable of assigning you as POA. It cannot be given to you by a demented adult. That would be conservatorship or guardianship.
You say "I have spent close to 15,000 saying your home from disclosure" . Do you mean you spent YOUR money? To save HER home? WHY. The POA spends HER money to save HER home.
Now you are speaking of recouping. Did you keep meticulous records of what was spent on her home??? Every penny and all receipts???
You need to see a lawyer because I am thinking you are not fully understanding what powers you have, who conferred them, and I don't know what you have done with the question of "saving" this home. I think you need professional advice.

Did your mother know what she was doing in designating you her POA. Was this a willful decision on her part? Some people can function near normal for years after getting an initial dementia diagnosis. Getting a diagnosis doesn't mean she immediately becomes incompetent and can no longer make rational decisions. If your mom was still competent when the document was signed, it is valid.

When it comes to a POA, if Mom knew what was going on and signed in front of a Lawyer, witnessed and notarized, I see no problem. Your problem may be if someone contests it.

Is mom going to need Medicaid within the next 5 years? Is she on it now? No, you probably won't be allowed to recoup the 15k. The house will need to be sold at Market Value. The proceeds going to her account and only used for her. If she is not already on Medicaid, you could try to recoup but you better have good records showing everything spent to total that 15k. Not just u wrote a check but materals used etc. Then pray when Medicaid looks at the closure paperwork they don't question it. Maybe you can put a lean on her house so you get the money when it sells. May be wise to have a lawyer involved who knows medicaid.

Like Alva, I'm not able to understand exactly what's taken place, or what your intent is.

1.   How did you "save" the home from foreclosure?  Was it in foreclosure when you began handling your mother's finances?  If so, where did the $15K come from?  Your mother's or your assets?   And why was it in foreclosure?  Was your mother unable to make the mortgage payments?   Or are these part of the $15K you've spent, i.e., making the payments to keep the mortgage current?

2.   Has the $15K included costs for her care?  

3.   How do you plan to "recoup" the $15K?   From sale of the house, or some other assets your mother owns?

Explanations can help respond to your queries.

I guess my big question is: who might dispute your authority? Are there siblings with a bone to pick with you? (Mine would). I don't think any agency like a bank or whatever would question it, because they wouldn't know that she had dementia. HOWEVER, when you go to sell the house it gets very very tricky. Some POA's and living trusts have special rules in them about proving she can't use the house. Some real estate title companies STILL want a lawyer to write something else (ours had to and it cost big bucks)...even though I was both the POA and the trustee.

I asked this question at one point and an attorney explained even with a dementia diagnosis, if the person has testamentary capacity at the time of signing, then a legal power of attorney can be made. My aunt went in and out of lucidity however, was clear about her wishes. The reimbursement is another issue that more likely an attorney can answer more specifically as well.

I asked an attorney about a dementia diagnosis and legal documents and he explained that a person must have testamentary capacity at the time of signing. This does not necessarily exclude someone with dementia. For example, my aunt was lucid for the most part and consistent with her wishes. If the power of attorney includes decisions regarding real property, then the agent can make decisions to benefit and pay for care for the elder. Record keeping is important if questioned at any point.

DONNALEE21,
I HOPE THAT WHAT I HAVE LEARNED RECENTLY WILL HELP YOU AND OTHERS WHO ARE OR ARE NOT FAMILIAR WITH HAVING A LEGAL DURABLE POA FOR FINANCES FOR SOMEONE AND WHEN IS IT LEGAL VERSUS ILLEGAL WITH REGARDS TO DEMENTIA/ALZHEIMER'S DISEASE. WHO IS IT THAT MAKES THE DETERMINATION OF COMPETENCY...THE PATIENT'S DOCTOR OR A LAWYER OR BOTH?
THERE ARE NOW 7 STAGES OF ALZHEIMER'S DISEASE. YOU CAN FIND THEM ONLINE WITH THE MAYO CLINIC, WEBMD, AND THE ALZHEIMER'S ASSOCIATION. THERE WERE ONLY 3 STAGES IN 2002 WHEN WE GOT THE DIAGNOSIS OF EARLY ONSET MILD TO MODERATE A.D. I NOW BECAME HIS CAREGIVER.
WITHOUT THE HELP OF A NEUROLOGIST, I HAVE LEARNED AS MUCH AS I COULD ABOUT THIS DISEASE, AND YET, IT IS STILL NOT ENOUGH. NO TWO PATIENTS WILL MANIFEST THE SIGNS AND SYMPTOMS ALIKE. SOME WILL BE GIVEN A CURSORY EXAMINATION BY THEIR PCP DR'S AND THEN THEY THINK THEY CAN USE THE PHRASE MCI AND CHANGE THE DIAGNOSIS NAME. ONLY A NEUROLOGIST WITH THE HELP OF A GOOD NEUROPSYCHOLOGIST CAN CHANGE THE NAME OF THE DIAGNOSIS.
BUT WAIT A MINUTE! SOMEONE ELSE CAN CHANGE THE WHOLE DYNAMICS OF THAT DIAGNOSIS. THAT IS JESUS CHRIST...THE ONLY ONE WHO CAN CHANGE THE NAME AND CALL IT "HEALED".
BUT UNTIL THAT HAPPENS, I WILL STICK TO WHAT THE ORIGINAL MEDICAL DOCTORS CALLED IT.
I HAVE READ INFO FROM LAWYERS FOR ELDER LAW THAT SAYS IF A DEMENTIA PATIENT IS LUCID AT THE TIME OF THE SIGNING OF A POA DOCUMENT...AND I ASSUME THAT IT COULD BE ANY KIND OF A LEGAL DOCUMENT AND NOT JUST A POA...THEY MUST BE LEGALLY COMPETENT TO READ OR TO HAVE THE DOCUMENT READ TO HIM OR HER...THEN ALL THE PARTIES CAN SIGN AND THE NOTARY PUBLIC WILL WITNESS THE SIGNATURES. ACCORDING TO ONE ELDER LAW ATTORNEY...HE WAITS 15-30 MINUTES AFTER THE PATIENT HAS SIGNED THE DOCUMENT, HE WILL THEN ASK THEM DO YOU REMEMBER SIGNING SOME PAPERS? AND HE WILL ASK THEM TO TELL HIM WHAT WAS THE PURPOSE OF YOU SIGNING THAT PAPER, TELL ME WHAT WAS IN THAT FORM YOU SIGNED.
IF THEY COULD NOT REMEMBER ANYTHING ABOUT THE REASON FOR SIGNING THAT FORM OR DOCUMENT, THEN THEY ARE NO LONGER LEGALLY COMPETENT TO DO SO. THEREFORE, THE FORM OR DOCUMENT CANNOT BE USED TO SET UP A Q.I.T. OR QUALIFIED INCOME TRUST-AKA MILLERS TRUST. WITH THAT BEING SAID, IF YOU FOLLOW THROUGH WITH COERCING OR TRYING TO ENCOURAGE A DEMENTIA PATIENT WHO HAS SUCH MEMORY LOSS THAT THEY CANNOT TELL YOU WHY THEY SIGNED THAT DOCUMENT, THEN YOU ARE A PARTY TO COMMITTING FRAUD AGAINST THE STATE. THAT IS AN ILLEGAL ISSUE AND IT IS AN UNETHICAL ISSUE. IF SOMEONE ASKS YOU WHO WOULD GO TO COURT TO CHALLENGE THE LEGALITY OF DOING A POA FOR THE PURPOSE OF PUTTING ALL OF YOUR MOM'S INCOME INTO THAT QIT TRUST...YOU WILL NOT BE ABLE TO GET ENOUGH FROM IT TO PAY THE BILLS. DO NOT ALLOW SOMEONE TO COERCE OR TRY TO PERSUEDE YOU INTO DOING SOMETHING THAT COULD BE ILLEGAL AND UNETHICAL IN THE EYES OF GOD. GOD'S LAW OUTWEIGHS MAN'S LAW EVERY TIME.