After the most recent update k9mail cannot fetch IMAP subfolders. No reaction to “fetch next N messages”
Pretty sure this reply (as well as some others throughout the last couple of months) should help 
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Hi Anton
I’ve just tested fetching the “next n messages” from subfolders on v13 and beta 14.03 and both worked successfully.
Does your mail provider use Imap path prefixes. If so, did you make any changes to those settings to resolve the v12 bug, assuming you upgraded from v12.x to v13?
I’m referencing the settings found at: Settings > Account > Fetching Mail > Incoming Server
- Auto-detect IMAP namespace
- IMAP path prefix
If the above is not applicable, i’d recommend enabling the app’s debug logging, replicate the issue, then review the collected log. See if there’s any error messages that stand out during the sync.
Works with a caveat - you need to delete and recreate the account with autodetect disabled from the start.
Changing the settings on an existing account does not work - something is cached somewhere.