Automatic synching doesn't work

Hi, I have a Xiaomi Mi Phone with Android: 10 QkQ1.190910.002.
Since I updated k9mail client to latest version it doesn’t work reliably.
Previous version was 100% working!! Since k9 version 5.806 it sometimes fetches mails, sometimes not. I start my smartphone every morning automatically at the same time. Two hours later I look for mail in k9. No mail found. I do the synch manually and mails are being fetched and when looking at timestamp I see that these mails were sent 2 or 4 hours earlier to my mail account and k9 would have to fetch them at start of my smartphone.But doesn’t.
During the day mails are SOMETIMES fetched from mail account. Sometimes not and only when I do it automatically.
What could be wrong or is it possible to downgrade k9 client to previuos?

I’m not certain if it is related, but starting with (I believe) 5.804 push stopped working with my primary mail service provider. Later versions (through the current 5.9) still work with gmail on these devices. I have backed up to 5.803 on some devices and push (again) works with my MSP, so will be moving forward on versions to confirm where it breaks (and will get logs and file a bug report).

You can’t “downgrade”, but you can export settings, remove K9, install a different/older release and import the settings. You’ll have to reenter your passwords. [and the “preview lines” setting doesn’t seem be maintained on export.]