That may be due to the compression settings.
The distortion is due to an overdriven guitar that is sitting pretty far back in the mix. It only comes in during those parts. I’ve noticed the same thing as well, i.e. that it detracted a bit from the overall mix.
I am, for the record (pun intended…guffaw), using the K-14 scale for final mastering.
I just redid the exercise, because there were some outstanding leveling issues. I adjusted this in the original mix, re-exported the raw WAV, then tweaked the compression and limiting settings before reprinting a new mastered version. This has been uploaded to Box at the same link above.
Have another listen and see what you think.