Some emails vanish suddenly

I use K-9 on my smartphone (Galaxy A6+) and on my tablet (Galaxy Tab A 2018) and many devices before (K-9 user since at least 2010). Since several months some (but not all) emails vanish after they have shown up as notifications. If I search for them they will be found and can be loaded and viewed. All other mail clients - Thunderbird, Seamonkey, Apple Mail, Roundcube - don’t show this problem at all.
I already tried to delete cached data of the K-9 app and even uninstalled and reinstalled K-9. Strange is also that the problem does exist on my smartphone and on my tablet. This points IMHO to K-9 as the culprit.
All mail accounts are running on a mailserver with postfix and dovecot running on FreeBSD 12.1 with OS and all ports up to date.
Any hints what can cause such problems?
Thanks in advance for any help!

I’ve installed yesterday FairMail as an alternative mail app. And FairMail shows all mails which K-9 refuses to display. Therefor I conclude that K-9 is the culprit.
Is there something I can do to debug the problem?

There are a few settings that could prevent messages from appearing. The “local folder size” and “sync messages from” can both cause this. Also, if you have the poll frequency set to never and don’t manually sync, some messages may get missed.