I was hoping for a solution so I could keep using the native mail app, but oh well... I installed K@ Mail and all the IMAP issues are gone. I think K@ is experiencing some stability issues, the app closed unexpectedly a few times on me. I think that's why they are offering it for free right now. Other than that, I really like what they are trying to do with the mail client. Very nice interface, and highly configurable.
I'm surprised the native client is such a major fail, but K@ seems to be a good alternative.