If the "Date " column is active Thunderbird displays the most recent message either at the top of the message list or at the bottom, according to your last choice for the "Date" column in the Sent folder. Click once on the Date column header to reverse sort order from ascending to descending or vice versa. TB will memorise your choice.

The "Date " column is in fact active. But no amount of changing the settings resolves the problem, and it only on the M1 MacBook Pro, as I said above Thunderbird behaves perfectly on my 2018 Intel Mac Mini.

But no amount of changing the settings resolves the problem..

Which "setting changes" do you mean?

Just an idea: R-click on this Sent folder > Properties > General > Repair Folder... Custom column sort order is saved in the *.msf (index) files in profile

I tried the R-click on this Sent folder > Properties > General > Repair Folder... didn't help.

And I need to repeat that this only happens on the M1 MacBook Pro, and not on my 2018 Intel Mac Mini.

I can't explain why this happens on one Mac and not on the other. When you switch to the Sent folder do you really see Thunderbird put the scrollbar "in the middle of nowhere" instead of moving it either to the top or the bottom of the thread pane, complying to default behaviour? To find the error I'd need a screen video capture or have to sit before your Mac and do a test.

"To find the error I'd need a screen video capture or have to sit before your Mac and do a test."

Not knowing where you live I'll select the option to send you a video. When I clicked on your name above it said something about sending a 'Private" message. If that will accept a video, I'd rather do that then posting the video here in the forum for all to see. In any event, I'll do it a bit later.