It was previously suggested to me that to stop the threading, to change the subject in the reply. This does NOT work, regardless of how many times a subject is changed. (I use Edit as New Message.)
The threads get so long that the message titles can't be seen in the email list - in the 1st screenshot, what looks like a Blank area in the Subject column is not blank; the list continues with that column out of sight. The second s-shot is to show the changing subjects.
I have looked for settings, can't find any.