As "detaching" actually involves removing a huge amount of text in the body of the email, I find nothing odd about yahoo deleting the original. What I find odd is that the new email sans attachments is not added.

Good point. Sometimes I see a brief 'blink' as the large message is replaced by the trimmed one. A bit of testing seems to indicate it's consistently messages in the Sent folder which disappear on removing the attachment. Inbox messages seem to get updated as expected. I wonder if you think it's not surprising they don't get moved to Trash, but just vanish.

Well, I guess it's not really a Thunderbird issue at all, except that I can't test it with Yahoo's web interface because it doesn't have an option to detach. The Yahoo server seems to be misinterpreting the commands from Thunderbird, but only for the Sent folder.

Trash is actually something of a fiction. Messages get copied to the trash and deleted in their original locations. So when it comes to "internal" types of actions I do not see it as strange that the deleted item does not go to the trash. Rather pointless making a full copy of something the user has just taken action on to reduce in size.

I would suggest you archive your sent mail before you modify it. It would make a simple method to work around the behavior and none of the detached messages would remain in the sent folder.

Your comment about 'internal' actions is the core of my interest in this problem, though at this point it's academic.

I have only a superficial understanding of how a mail server actually works, and how a client interacts with it. I suppose that the 'human-friendly' folder structure is also a fiction. It just seems odd that Yahoo's server would respond differently depending on the type of message (sent, received) or folder designation (sent, inbox). I'd guess that for those sent messages the server is ignoring or misinterpreting a command (request?) from Thunderbird.

I appreciate the suggestion to archive before a 'destructive' operation, but I might like to learn a bit more about the back end of communication between client and server, in case you know of an introductory resource.

I'll mark this 'solved' even though the behavior still puzzles me.

I just had this issue today. I use Thunderbird to retrieve my Protonmail email and it also removed the full message when I detached the images attached to the email. The original email is not showing up anywhere (inbox or trash).