Steps to reproduce the issue

1. Create some custom user fields in Users>Fields
2. Within a user plugin, call JUserHelper::addUserToGroup($uid,$gid) inside of the onUserAfterSave method
3. Edit and save the user

Expected result

Groups change, field values remain

Actual result

groups change, field values are defaulted

System information (as much as possible)

J3.7.5

Additional comments

I created a CLI to test this and it works as expected. This only seems to happen if I add or remove groups within a user onUserAfterSave event

My temporary solution is to not use JUserHelper to add or remove groups, instead I'm querying the table directly to make my changes. I would rather not do this, but it keeps me moving forward:

	$db->setQuery('DELETE FROM #__user_usergroup_map WHERE user_id = '.$id.' AND group_id IN ('.implode(',',$removegroups).')');
	$db->execute();
	
	if(!in_array($addid,$user->groups)) {
	    $o = new stdClass();
	    $o->user_id = $id;
	    $o->group_id = $addid;
	    $db->insertObject('#__user_usergroup_map',$o);
	}