This morning, I noticed that all my IMAP accounts had lost all their folder associations in Settings / / Folders. Also, 2nd class folders that had local content yesterday evening were empty. This affected eight IMAP accounts across four Android devices. K-9 was last updated in May, so I suspect an issue with my IMAP provider. They did not see anything odd at their end. My best guess is that K-9 was able to connect to the IMAP service, it returned no folders for a period of time overnight, then started working again. The 1st class folders were rebuilt but the I had to manually refresh the 2nd class folders. Does this make sense?
The server returning an empty folder list would explain the behavior you were seeing.
Thanks for the confirmation. Unfortunately, my IMAP provider claims there were no issues that day. Yet another glitch in the I/T universe.