After update to 6.400 my app started to reload all messages in my inbox imap account. It seems that it does something on the server, because my Thunderbird client does the same after it happens on k9mail. I have upgraded also to the latest version 6.5xx and behavior is the same. I have done a fresh reinstall. My server is Icewarp.
I have noticed, that this behavior becomes a bit less often if I set “push folders” and “sync folders” setting to “all folders”.