Has anyone had a loved one go “downhill” after moving them to assisted living?

Are you looking for people who have a 99 year old parent to write to you that the parent is thriving? Because no matter where that parent lives, I think that you will find they are unlikely to be thriving. Your mother asked to move to a safer place because she was cognizant of what is coming. I am 80. I am knowing what is coming now. Mentally. Physically. And I am knowing I will have to move to ever increasing levels of safety for the good of myself, and for the good of my family, who should not have to make their lives all about ME.

I don't know how many will write you to say their century old parent is thriving. But I have been some years on Forum, and I doubt it will be many.
My own father, nearing his mid 90s, told me he was exhausted with life and longed only for the long- long nap as he called it. He said it was so difficult for him to get out of bed and participate. He had had a wonderful life; he had no complaints, but he longed to "go".
Many of my patients when I was an RN felt the same. They told me that they could not tell their families this because their families would not hear it, would negate every word they said. So they were left to share their truth with a virtual stranger, their RN.

You lovely Mom will not complain to you. She will likely go just as she has lived. With grace. You are so lucky to have had her all this time. My heart goes out to you BOTH.

She is 99, what would you expect to happen at that age? Be it at your home or in AL.
I really doubt that it is temporary memory thing, the shade has started to draw down, nothing unusual about this at age 99.

My mother is 98 in AL, her mobility is dwindling, she is tired and at times confused, nothing serious just moments of confusion.

Her thriving days are over, it is now about safety & comfort.

Nothing in life remains constant it either gets better or worse.

Accept the decline, obviously she has.

My mom declined with the move. She was at a deeper stage in dementia that it sounds like your mom is. I am not sure if the move caused the decline (changing environments is certainly stressful) or if she was already confused and hid it well due to her familiarity with my house. I suspect a little of both. She was so familiar with my house that things came easy by routine. When she moved she had to remember and think about every function and that was stressful for her. She did not adjust well. She did not like so many folks coming into her room. She was an introvert and wanted to be left alone which is impossible in her condition. She did finally begin to enjoy some things and was there about 5 months and passed in November. I suspect congestive heart failure even though we are not sure.

I have written before that I believe personality plays a role. I saw many folks adjust very well. They were social and enjoyed having new friends. That was not my moms nature and that made it more difficult. She would have been happy had I been able to stay every minute with her. I was familiar and made her feel safe.

Give it time. Your mom is going to meals. (my mom would not). She may find friends that can give her new energy. I wish you well during this difficult journey.

A decline is common with any move. It could have been to a siblings house, it could have been moving from one room to another in your house.
Once a decline occurs it can take a long time to get back to "baseline" if that happens at all.
At this point I would suspect that her cognition is going to continue to decline. That could be due in part to age as well as the move.

I think folks "thrive" in AL when they move when they are younger (think mid 80s) and more resilient.

Safety and comfort for her as well as free time for you. Last leg of the journey, don’t second guess yourself and agonize over the specifics.

God Bless your mother at 99. She has lived and thrived in the past but has declined enough to want more rest. It does take longer to adjust from a move, a major stress event but hopefully not much longer. Like in infancy, comfort is vital.

I am sure the move is the issue. When my dad turned 99 his doctor and others he interacted with were all amazed how he was at the top of his game as they described his mental capacity, mobility wise no. I imagine the PT is to much for her at her age. My dad said no more PT and PT agreed his knees were shot and PT hurt making it exhausting. My dad in his last weeks said I guess I will need to go into a AL living now. I said why? He said because my mobility is not getting better. I told him he was not bed ridden yet and with the sit to stand moving him from bed to wheelchair etc was easy. He made it 6 weeks after he turned 99. We kept him in his home. His last six weeks I slept at his home during the night but was not there all day. The night before he died was the first time he showed any confusion. He said something that was off the wall. The next morning I could not stir dad. If your mom is slipping now she is in AL. Be prepared. Maybe stop the PT and see how she does.

When you have reached that many years in life, it’s normal to not adjust to a new situation. Before moving, she felt secure in knowing what each day would be like. Her surroundings were familiar with her things and family, she knew the foods she would have that day, where her tv was, her bathroom, her clothes, etc. She knew her reality and routine. All that has changed and she has to develop new ways in her brain to remember all this newness. It’s very exhausting, challenging, stressful, and a bit scary. Some people adjust and others never do. I hope that it gets better for her. Best of luck!

To be blunt she will unlikely "thrive" ...because she's 99. My mother is 98 and finally we had to move for her to assisted living last July, and she is now on hospice level. Though she is mentally relatively alert, her memory is declining. Yes the move was disconcerting for her, but the fact is at 98 few are thriving. She can't do things for herself, she can barely see, she can barely hear so she can't participate in conversations, she can't walk past a few steps, she can't go to the bathroom by herself, and she needs assistance with...everything. This does not make for a fulfilling life for most people. My mother had lived long past a natural life span, and everything is starting to go, naturally. She was in PT but complained about it a lot as it was hard for her, and because she has congestive heart failure etc they stopped the PT and she just goes to exercise class now and then. I think the PT was hard for her because it seems pretty ridiculous honestly to expect her to "improve" at 98. As I am 62 and she's my mother I have a bit more emotional acceptance (I understand more the pains and difficulties of getting older then I did 20 years ago), and I'm ready for her to go.
Your grandmother is in the right place, and she is lucky to have such a loving caring grandchild. We should all be so lucky to have one person at least who doesn't want us to go. But at her age she's not going to thrive. Just keep loving her and that is the best support you can give her.