Adding a new library to the global library table will happily use KIPRJMOD by default

Adding a library to the library table automatically replaces the file system path with path variables if possible. This is generally a good thing. But the use of KIPRJMOD in a global library table will most likely be a mistake by the user. Meaning simply using it without asking any questions might not be the best idea.

Example: The user makes a new library using the library editor. It by default points the user to the location of the project. New users might simply create the library that way and select "global" in the next dialog (as they do not know what global/local means). KiCad will then use KIPRJMOD in the path without the user ever being aware that they might want to change this. (Which will lead to an error message when opening a different project.)

There are two options from this point. Adding a project local library to a global library table could print a warning message that this is not considered a good idea or kicad could use the full path instead of path variables. (I lean towards the former as the most likely case here is a misunderstanding of kicads library system.)