I think the whole thing with Ageha remembering stuff is a bit more complex..
After she killed herself and he tried freeing her again, it didn't seen like it was actually Ageha but rather a completely different person. Some are saying it is just a different personality, but I'm thinking that maybe it could be something similar to puppet sword.
For example if she had a second soul inside her body that was able to remember things despite the reset, even if vaguely, and that soul can either influence her or take full control, or do nothing at all. That could also be the reason to why she was sealed, like if she was at risk of losing control.
This could also mean that there are conditions which allow the MC to free her in specific states, since he has seen at least 3 different ones iirc (besides the times she escaped by herself).