Blizzard have detailed changes to M+ rating for Dragonflight, as keys above level 10 will be grating increased rating, as explained below.

Mythic+ Rating (Source)

As we previously noted, we’ve increased the difficulty of Mythic+ dungeons at keystone 10 and higher in Dragonflight, to better match the quality of rewards offered by those difficulties.

Following testing and feedback, and considering how much players enjoy pursuing Keystone Master as a seasonal goal, we’ve decided to make an adjustment to grant additional Mythic+ Rating for keys above Level 10:

M+ keys at each keystone level from 11 and up are now worth an additional 2 points of Rating. For example, in Shadowlands Season 4, completing a dungeon exactly on time on both Fortified and Tyrannical would net you a Rating of 250 for that dungeon. In Dragonflight Season 1, you will now earn 270 Rating.

This means that while keys above Level 10 will become more difficult and you may not be able to push as high as you did in recent Shadowlands seasons, you’ll achieve Keystone Master at lower keystones. We expect players who previously earned this achievement to find the challenge about the same as before.

Thank you very much for your testing and feedback!