There seems to be a bug that might be rendering one (or more) of the fragments unobtainable when triggered.
the one-shot flashbacks can sometimes end simply by switching rooms.
(and I already knew of the one you spoke of)
May you elaborate on this a bit? I've played through multiple times and I've been able to obtain all the memories and fragments. If you initiate the sequence near the end where you follow the feathers through the levels, the memory fragments become locked off, but I can't think of another reason why you wouldn't be able to obtain them? It could just be that it's pretty hard to navigate the world since there's no map, and the level changes depending on whether you're in the present or the past. I know I had trouble finding them all the first few times. If you can explain what you think is wrong in more detail (i.e. whether it's in the underground or the castle and whether it's in the present or the past) and provide a screenshot, we'll take a look into it.
https://www.dropbox.com/s/6lb1e0hynbiylff/20190616_174840.mp4?dl=0
this should show one iteration of the bug I spoke of, in this case also making firefly unobtainable.
This bug is fixed on the last version on the game available here : http://www.mediafire.com/file/uvp2j226773aopn/Memento.rar/fil