Ion Hazzikostas has revealed in an interview with EU Content Creators that keystones will drop by 10 levels at the start of Dragonflight Season 2. This means that you will lose 10 levels of your maximum key you done in the last week of Dragonflight Season 1.
This means that if you time a +17 next week, you'll receive a +7 in the first week of Season 2 In an interview with EU Content Creators, Ion Hazzikostas has stated that Mythic Keystones will be depleted by 10 levels in at the beginning of Season 2 of Dragonflight. Learning a brand new set of dungeons comes with a steep learning curve, so players will have to push their keystones again.
This is a change from past Mythic+ seasons, in which keys would usually deplete 3-5 levels on a new season rollover. This ensures a fresh start for all players in Dragonflight Season 2, as they learn the new dungeon rotation.
. Read more on wowhead.com