Hello. I use K-9 Mail since many years. What I still experience from time to time is:
The spinning arrows in my Android 8 status bar are rotating and do not disappear after a short moment. This is combined with a heating up phone.
Sometimes it helps to turn WiFi off and on (so I enforce switching to a different Internet connection), but sometimes this does not help.
It also helps sometimes to terminate all apps, but this also helps only sometimes.
Finally I have to restart the entire phone, otherwise the phone gets really hot and drains battery.
I am pretty sure this is caused by K-9 Mail itself, all other things still work.