Problem #1:
My wife’s MBPro (M4, 26.6.1, 24GB, unknown version of SpamSieve!) displays “All Messages” message list “jumps” every 30 seconds (timed and verified).
Not a big problem, but very distracting. Wife not happy!
Talked with Apple Support while “Safe Booted” with no changes in the behavior.
Finally remembered the SS was installed! Disabled it, Uninstalled it. Deleted it. Problem fixed! Will try to contact SS dev.
It’s normal for SpamSieve to check the inbox repeatedly. It’s not really clear to me from the screen recording what’s “bouncing.” Is Mail adding new messages to the list? Are they spam or good?
You can check the Log window to see exactly what SpamSieve is doing. Is it processing messages that you didn’t expect?
The main thing I see is that you have a bunch of blue messages, which it looks like SpamSieve identified as spam but Mail failed to move out of the inbox. (This can happen if the Mail settings aren’t quite right.) Or perhaps it moved them and then something else moved moved them back
By the way, you don’t have to uninstall and delete SpamSieve for testing purposes. You can just temporarily disable filtering in the settings:
Thanks for such a quick response!! 
The “bouncing” is when SpamSieve seems to “move” a message to Junk or Spam and then ‘fails’ and puts it back into the original Inbox. It does this with every message that is UN-read. Each time a message is temporarily moved out of the Inbox, that list shrinks a bit causing items BELOW that message to move up. When the move fails, the message is put back into the list and the items list return to their original positions (moving UP or DOWN as needed.
I don’t know why there are “a bunch of blue messages”! 
At first I thought you were referring to the “Unread” marker. Clearly that is not why they are blue.
Bottom line. Deleting and re-installing the app has stopped the distracting ‘movement’ behavior.
I have another problem but will search for a current thread or start another…
This is what I was referring to above about Mail failing to move messages.
Presumably, it’s because of the spamminess score that SpamSieve assigned.
Since Mail is responsible for moving the messages, reinstalling SpamSieve is not going to change whether it’s successful at moving them or not. So perhaps there was a server issue that was coincidentally fixed around the same time. Anyway, I’m glad things are working now.