At my Edwin's most recent level up, he didn't gain any spells. In fact, some spell slots disappeared from his book... would this happen to be a known bug? I can't figure out any logical reason why it would happen.

Hmmh, weird... Does he still have his family amulet, or was it somehow removed? Seeing it gives some extra spells, that could be a reason for those dissappeared spells. Although it IS irremovable anyways...

Did he level up to 10? It is a well known bug, yes. Do you have the official patch?

This topic can help you! The person had the same problem as you, apparently:

http://www.sorcerers.net/cgi-bin/ultimatebb.cgi?/topic/10/5170.html

Hope this helps! ;)

It's because when you get him he starts with more spells than normal

Yes, it's a bug--some versions of Edwin come with more spells than they should, so he loses spells on his first level up. Fixpack and Baldurdash both fix this.