If you call get_user_capability_contexts() with $getcategories as true, and no category sort order:

get_user_capability_contexts('tool/myplugin:viewdetails', true, false, '', '', '', '', '', 1); 

You get undefined variable error is thrown:

PHP Notice:  Undefined variable: orderby in /opt/homebrew/var/www/moodle/lib/accesslib.php on line 4250 

This happens for categories and not courses because in the category section, we use a different variable, $orderby, to build the query, while in the course section, the original parameter variable is used - so in the empty cause, it is just used.