The issue looks like that. I always keep emails as read, very rarely as unread; and the behaviour is strange - I mark emails as read - all of them. Then they are kept as read untill a new email arrives - last ones which I have marked as read suddenly become unread. I can’t understand why is that - looks like a bug.
P.S. it doesn’t matter if I mark it within the specific mailbox or connected one.
Nevermind. Titan email is unfortunately garbage. It’s an issue on their side and not on K9mail application. The ticket can be closed.