Account only syncing top level folder

No matter what number I put in the setting for folder levels to sync K-9 Mail only syncs the top level folder. This means that when filters are run on another machine I don’t see the emails on my phone. What is this setting supposed to actually do?

Using IMAP and POP with multiple accounts makes navigating them on the app a nightmare so with the IMAP accounts I only use POP access on the app but never see any messages as Thunderbird filters means they’re not in the inbox when K-9 Mail checks and it doesn’t seem to check any sub folders as the setting would suggest it should.

Actually forget this. Just looking through the forum and the response from @cketti on anything vaguely related to POP3 is to always dismiss POP3 as not working when in fact I have never had an issue across multiple providers and POP3 still very much works, it’s more reliable than IMAP and the only issue I have is with the app (which I would assume is because the developer doesn’t know how POP3 works) so it seems pointless to carry on using or supporting an app where the only developer making decisions is misinformed and arrogant.