I just met with a great-aunt who has been a bit distant until now. A rare time for me meeting with relatives of her generation since I ran out of living grandparents about a decade ago (though thankfully I at least hear from most of them from mom occasionally).

We talked about some of our common relatives of her generation and some have dementia-related anterograde amnesia not quite unlike the OP's mother. At least some of them can at least remember "new" things like the very fact that their memory doesn't work right, but a lot of it is down to luck and conditioning, and even then it's really hard to always consider that something might have happened hours or minutes ago, but you just forgot about it.

Memory is hella complex is what I'm saying.