From my previous post I noticed that I couldn’t re-connect to the Pi using screen after I’d disconnected using Ctrl-A Ctrl-D. Reading additional suggestions here, Ctrl-D is just a disconnect, it doesn’t end the process that’s attached.
Instead, use Ctrl-A Ctrl-\ and it will ask if you want to terminate processes using screen.
If you do detach, from here you can use this command to list processes attached to the serial port:
fuser /dev/tty.usbserial-11240
This will show a pid that you can kill, and then you can reattach again.