Has anyone ran in to false deja vu?

She cannot separate past and present. See a Neurologist soon.

I did a test on my mom - I brought something to her & said 'remember, you asked for it' - a small pause then she said 'yes I remember & thanks' - she didn't ask for it but it was my way of testing her memory - in other words I planted a memory that she agreed with

Now I know that anything she says must be checked out as to how real it is - mom would also take several partial memories & make a new one - fortunately on about the 3rd or 4th time, I witnessed what happened [my uncle's funeral] & quickly realized what she was saying didn't happen - she said my dad didn't go according to her memory but all 3 of us were there & she did a reading as part of service

Your step mom may know down deep her memory is getting bad so she may be trying to show you how good her memory is - much like someone who is drunk trying to show how sober they are - she may be worse than you know into dementia - have her seen by a doctor & contact that dr before & explain you concerns - this may get her help that she needs or at least give a proper diagnosis

Or maybe she needs new glasses?

There are more than 70 types of dementia. Alzheimer's is the most common. Talk with the doc. They would certainly have more knowledge. One strange types I have heard about is Semantic Dementia. They lose the ability to recognize/identify and name very common things. You couod try googling it.