Update.
I updated OSX to 15.4, and it still wouldn’t work.
Then I tried other emails, wondering if there was something about this one source that DT didn’t like. They got through. But I still could not get the one source through. It would make the little plinking sound, close the sorter, and I would then go check. Nothing.
I had the sorter on the right side of the screen, and was getting sick of dragging with the trackpad across the laptop screen, so I moved the sorter to the left side so it would be closer to my email inbox.
Tried it again…And voila. It worked and copied the full email to the inbox in the Sorter. But only for one email at a time. If I select several emails and drag/drop to the sorter, it may copy one, but generally the pattern is that it doesn’t copy any. I have to do them one at a time.
Maybe, since it worked on your machine earlier today, you might want to try a bulk import via Sorter?
The other emails I tried were also one-at-a-time. Maybe it’s an issue with bulk importing via Sorter? I’ll keep trying different scenarios and see if I can identify more useful patterns.
Needless to say, that makes no sense to me. I report the whole tale hoping that maybe it might mean something to you or your devs.
Further update 250407.2222
It seems that it doesn’t like the bulk copy function. I tried it with a couple other email sources, and no go. It will transfer one at a time, but any more than that and it chokes.
(And, of course, that may be by design. Though I don’t know what that would be set up like that. Other documents from Finder folders, etc., seem no to have issues with bulk importing through the Sorter.)