I have that too...forgetting things in mania, but TBH, I often drink during manic episodes too, so some of it is blackouts...drunk dialing conversations, etc. But not always. The mania makes you very in the moment, so it's easy to not store things in your long term memory. It's one flight of fancy to another and you have no idea how you got there, but you don't care.

This is my experience anyway.