I don't think it's a regression.
The issue is that a missing msgDB (.msf file) in an IMAP account isn't being rebuilt when TB is offline, even though there are probably local copies of all the messages which could provide the required info.
My understanding is that the IMAP folders treat offline copies of messages as kind of a temporary cache, not as the authoritative source. The IMAP server is considered the authoritative source. So when it goes to rebuild missing msgDB, it asks the IMAP server for all the message headers rather than parsing them from any offline-stored messages. Obviously this only works if TB can actually contact the IMAP server.
So in theory, in offline mode we could make an attempt to use local copies to rebuild the msgDB, but there'd have to be some reconciliation/sync algorithm, and I suspect that might get very tricky... even just figuring out which message on the server a local copy represents would likely have a lot of corner-cases to sort out (the local copy is missing assorted IMAP metadata the server provides, which is normally stashed in the msgDB).