Kmail shows read email as unread

Thanks to wise counsel from this kdepim list, kmail 5.9.3 is up and running as POP3 on my 
Debian 10 (KDE) system.  

Prior to making the switch, Kmail would regularly and erroneously show as "unread" mail 
that had been read a couple of weeks earlier.  That was maddening.

I reinstalled kmail on my new computer (everything same except new computer is faster) 
and was most relieved that kmail was not showing "read" emails as "unread".  However, 
now, about a week into my new computer, the problem has returned. Kmail properly 
shows new mail as unread but far too often it also shows read email as unread.  The read 
emails are almost always emails I read on one day a week or two ago.  For example, today 
kmail showed about 8 emails which I had read on March 22, 2021 as unread.

Can anyone help me figure out how to keep kmail from showing emails read two or three 
weeks ago as "unread"?

Thanks

John