Amnesia seems to make more sense than anything else. I'll admit I'm not familiar with anything that could have caused that in Amestris, but there are plenty of worlds where something like that could exist. It's curious that some of us are missing so much longer than others, but I suppose that could be a side effect.

[The idea of missing seven years' worth of memories is sort of terrifying and not something Isaac wants to really think too much about!!]