dovecot - Sep 2019 - Mail messages are occasionally getting "stuck" behind one message on the server with POP3

I'm currently using Dovecot 2.2.33.2, although I've had this particular 
issue happen periodically over the last few years with other Dovecot 
versions.? It seems to only affect accounts I'm receiving with POP3, not 
IMAP ones.

What happens is the "top-most" (or, oldest) message in the
"cur" folder
seems to get "stuck".? So I'll, say, come back in after a break to
check
my email (Thunderbird in my case, latest version), and in one of my 
folders I'll have 30 copies of the same email (all with the same 
timestamp), and no newer messages delivered since the timestamp of that 
message.

If I go look in the "cur" folder, it has lots of messages in it. The 
"new" folder is empty.? Every time I click "Get Messages" in
the mail
client, I get another new copy of the same email and no other mail.? If 
I go manually into the cur folder and move the top-most (again, meaning 
"oldest") message out (which of course corresponds to the message that
I'm receiving copies of over-and-over repeatedly) then when I click
"Get
Messages" all of the other messages come pouring into my mail client 
just fine and everything becomes "un-stuck".

I've done some experiments, and if I copy that same "bad" message
back
in to cur with its original filename, it gets stuck again.? It is an 
encrypted message, so I tried decrypting it and putting it back in cur 
with the original name, and it gets stuck in the same fashion even when 
it is decrypted.? I tried putting the bad message into an account that 
receives with IMAP, just to see if it would maybe keep messages from 
delivering or something, but new messages come into the IMAP account 
just fine, even with the bad message present, so the issue seems 
specific to POP3.

Any ideas what might cause something like that?? I've had it happen 
twice over the last two days (which is unusually frequent, hence my 
message here), and the message sources are varied.? Today it was a 
normal email message from a person, and yesterday it was a Twitch 
notification.? I don't see anything unusual in the logs (and I did turn 
on debugging to see if it gave some additional info), just normal POP3 
logins and connections closings when I'm clicking "Get Messages".

-David