Change Template Based on Client Group

[JLHC]

I created a hook to change the Template a client will see depending on which Client Group they are lumped into:-

<?php

function clientgroupTemplate($vars) {
	$client = Menu::context('client'); 
	$groupid = $client->groupid;
	if ($groupid == 1) {
        global $systpl;
        $systpl = 'MY_TEMPLATE';
        $GLOBALS['_SESSION']['Template'] = 'MY_TEMPLATE';
        $GLOBALS['CONFIG']['Template'] = 'MY_TEMPLATE';
	}
}
add_hook("ClientAreaPage", 1, "clientgroupTemplate");

However, the hook seem to only work on the second load, which means that the client will first see the default Template after login and only after a refresh or going to the second page will they see MY_TEMPLATE. 

I tried disabling all other hooks to rule out any conflicts. 

Is this due to caching like OPCache or is there a better way to write this hook? 

Thank you. 

Edited April 13, 2020 by JLHC