I agree that this would be a good idea, but it is not a bug in the current implementation.  The only place imaplib itself uses the cached capabilities is *before* login, in the starttls method, and there it refreshes it after starttls succeeds.

Although it isn't documented any more than the 'capabilities' attribute is (and therefor neither are part of the public API currently), you can call the capability command yourself if you need the refreshed capabilities in your own code.

Good catch on the bug in the IDLE patch, though.