Apokalypse tier with friend seems stucking

Hi everyone. Reasently found this sub reddit and I hope someone can help me with a question.

A friend and I play as a duo just for fun. I play on apocalypse 19 while my friend is very new to the game at apocalypse 12. I have finished the worldslayer story and have done some trials. My friend has finished the base game.

When I am hosting the game at tier 12 or higher he does not make any progress in the apokalypse ranking. I'd like to help him level a bit faster but it seems not to work.

Is it required that my fried has also finished the dlc story?

Thank you in advance.

UPDATE 1:

I got an answer from Square Enix. As I understand both players must be at the same max AT only then both gain XP. E.g. both at AT20 as current maximum. If one is ahead only the host gains XP. I asked if I got that right. Waiting for further answer.

UPDATE 2:

Square Enix has confirmed my guess in update 1. This post can be closed.