All emails have been received and synchronized in the background, but the software has not given any notice. You must enter the software to see new emails.
Are you using the beta version of k9? On my android10/lineages moto g5 I currently don’t get any notification on new mail any more, neither, with the current beta.
Make sure k9 is exempt from any battery optimization on your device and double check to make sure you’ve set notifications up properly.
On my OP 7 with OOS 10 I receive all notifications, same with my Samsung tablet running Bliss which is Android 10.