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.