Thanks for the investigation! I ended up here while researching the same thing. ServiceNow has decided to introduce a new "Save" UI Action specifically for the sys_script_include table, and sadly it is read-only.

Currently this makes it impossible to add small modifications to it, such as marking it as 'Form button' to always display in the form without having to create a new one and copying over the same script, having to manually check on each upgrade if the script has been enhanced on their side.... 😅