Traditional QB rating is a lot like batting average in baseball, it tells a story, but not a very complete one. You need to start looking at things like EPA and other advanced stats. They incorporate so much more than traditional rating which is just a simple math formula based on attempts, completions, yards, tds, and ints.
On an EPA basis Brock was worth 25.2 total EPA and 0.72 EPA/play against Seattle, his best game of the year, but this week he was worth 3.2 total EPA and 0.09 EPA/play, which is in fact his worst performance of the year and a lot closer to several of his other games than last week was. (From weeks 13 to 18, Brock was a top 5 QB with an EPA/play of 0.207. ) Mahomes has one or two games close to that low. The others have all had a least a couple below that, if not games in which they produced negative EPA.