Created attachment 125719 [details]
Message list

SUMMARY
Messages are sometimes always rendered as unread (even context menu show thinks they are unread since it offers option to mark them as READ, which has no effects) even when read and the count for unread messages beside the folder is 0 (not displayed)

STEPS TO REPRODUCE
Honestly no idea, it just happens.
I have set message view as:
Message List > Theme : Classic
Message List > Aggregation : Current Activity, threaded
I think the bug shows when the message is a reply to some other mail.

OBSERVED RESULT
Message rendered as unread when not counted as unread, while being unable to mark it as read. (The option is in context menu but has no effect)

EXPECTED RESULT
Message is properly rendered & marked as read.

SOFTWARE/OS VERSIONS
Operating System: Artix Linux 
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.66.0
Qt Version: 5.14.1
Kernel Version: 4.19.98-1-lts
OS Type: 64-bit

ADDITIONAL INFORMATION
Sometimes I use Android K-9 Mail to browse my mails when KMail is not active, than I mark the mails back as UNREAD for KMail to run filters on them as on new messages e.g. move them to different folders.