Hmm... I'm trying to understand - maybe I have missed something in your posts.
Those are still only messages that you have in the trash, correct?
And, the only messages that disappear are from certain users, and are already in the trash?
So, you have simply filed those away, and no longer need those messages - because you put them in the trash?
If all the above is true, but the fact remains that no messages from
other senders disappear from the trash - only from those senders?
Further - that messages from those senders do not disappear, unless they are in the trash?
Is all the above correct?
So, (I'm just curious
) if you really want to keep those messages, then don't put them in the trash.
Your personal workflow, with muscle memory involved, might be hard to change.
Maybe you can't fix the "why", but you apparently have the workaround of not using the trash unless you are deleting messages.
Delete messages (which moves them to trash), then
empty the trash, and you are
done. That gives you control over that part, anyway. The result: you solve the problem of certain messages disappearing if you leave them in the trash - because they are already gone.
OK, I accept that it's a "head-in-the-sand" approach - and you can ignore my post.