I disagree that this issue is not serious since there is a workaround. If the cache needs to be deleted every time you want to access your email, it basically makes the program unusable as an IMAP client. Also, sometimes certain emails do not appear in the inbox even after deleting the cache. They are just silently discarded, and the user might be totally unaware of the messages unless he accesses the IMAP account with some other MUA such as mutt.
Any ideas how to help debug this further?