Would you buy DLC for a timed third party game pass game though? A game which you can't know how long it will be on the service for? Like Desperados III or Injustice 2 or packs for Assetto Corsa?
All gamepass games are 20% off to buy digitally if you decide to keep it. I would buy the DLC, play with the intent of finishing before it is ever removed...but if I don't I still get to buy the game for less than if I had not been subbed to gamepass.
So as long as I am willing to buy the game at the discounted price, it is a winning scenario no matter how you look at it. The way I see it, if you are not willing to buy the game at a discount in the first place then there is no reason to spend money on the DLC anyway.