One day left on discount game

As stated you would have to get the games during the time period that it is free. Normally games that go on sale will be on sale again you just have to watch out for them.

If you miss the sale, it will probably come around again. That game always seems to be on sale.

Deals Unlocked ends 11:59pm on June 15th. (your local time)

You can find all the games on sale here https://www.xbox.com/promotions/sales/deals-unlocked/xbox-games

This differs between how many days til game is released

or how many days left til promotion/ Sale/ discount or the game itself remains

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.