Module: EVENT_AFTER_SAVE_ELEMENT won't detect changed user group
That's because groups are not attached to the user directly. At the moment Craft stores the user it hasn't handled the permissions not the user groups.

The workflow is


Handling all kinds of variables and data
Fetch/Create a user
Populate the new field values for the user
Store the user
Handle verifications such as sending them an email
Assign the user to groups and grant them permissions


At the moment your event triggers the user wasn't assigned to the groups.

If you want to change users depending on their groups you should use the  Users::EVENT_AFTER_ASSIGN_USER_TO_GROUPS event