Game Pass Discount

So just to check what you are saying in regards to the discount, would I have been able to buy the game discounted while it was still on game pass, but because it had just left there was no discount?

Yes, the game still has to be on Game Pass in order to have a discount.

The discount for purchasing is for games in the Game Pass catalogue and wouldn't work for a game not in Game Pass.

I think games typically do leave Gamepass at the end of the month. That game could have been an exception?

As for Gamepass discounts;

How do Game Pass discounts on game purchases work?

Xbox Game Pass members can save up to 20% on Xbox One games purchased from the existing catalog, as well as up to 10% off related game add-ons and consumables.

Discounts are based on the Microsoft Store price and on the game’s global launch date. Between 31 and 90 days from a game’s launch date, members receive up to 10% discount off the Microsoft Store price. After 90 days from a game’s launch date, members receive
up to a 20% discount. For game add-ons and consumables, Xbox Game Pass members receive up to a 10% discount from the day the add-on is released.

Game Pass discounts are not combinable with other offers and are not redeemable for cash; discount offers exclude titles within 30 days of launch; discounts are based on Microsoft Store price. Requirements, features, online services, or subscriptions are
subject to change and/or retirement. Discounts are subject to the Microsoft Services Agreement.

If you’re using a desk, you will not get a discount on the wall condition as that discount is based on digital ownership

No call of duty game is on Game Pass

It's to show that you can get a discount on them with Game Pass, not that they're included with Game Pass.