I don’t know if this is supposed to happen but after testing the Ctrl+M (turning scriptwriting mode on then off), I find that the Script mode - Screenplay option is greyed out (screenshot attached).
This feature will only become available again after you open another document from the binder then go back to the original document.
I’ve found that if you turn script mode on, the script mode option remains available. You can move to other documents, then come back to the script, turn off script mode and the script mode option will become grey again until you open another document.
I then tested it in split screen mode. If you turn script mode on and off (ie. the script mode is now greyed out) on one of the documents that are showing (I am in the vertical split screen mode and I was on the document on the left hand side), then just select the other document and come back to the original document the script mode option will become available.