Without details, and better yet a log, I doubt you’ll get much help.
K-9 version, device, Android version, email provider, whether POP3 or IMAP are needed. Also check your settings to see which Inboxes are set to go to the Unified one.
I use K-9 on 4 OnePlus devices (all with IMAP of course), stock OOS + custom roms, A/OOS11 to A14, and on all of them the Unified Inbox syncs as it should. So I’m almost certain it’s not a K-9 but a setting issue.