In short, the $FULLSCRIPT global is missing from the list of allowed variable names.

As of August 22, 2024, here are the allowed globals:

While I see the PHP superglobals ($_GET, $_POST, etc.) and the main Moodle global objects ($CFG, $DB, $USER, etc.) there are a few Moodle global literals like $ME, $FULLME, $SCRIPT that are present.

Today was my first day seeing those literals, because I was looking for origin of $FULLSCRIPT. I don't know the difference between the four and I don't see any uses in core of $FULLSCRIPT outside of it being defined and initialized. Someone else didn't know, so they incorrectly lowercased the variable thinking that it was a plugin-specific variable, which may have caused some issues.