Source: WowAce

Should affect the pet(s), as well. Needs testing to see if it only fails in M+ or if regular (+0) difficulty is also affected.

The detection method is currently set to NPC (loot events), which obviously won't work in the keystone runs. Unfortunately, there appears to be only a statistic for Heroic difficulty being llisted on wowhead. This means the way Rarity tracks attempts for the other other keystone dungeons won't be available for this one.

It appears the Junkyard tracking does work, but only if the difficulty is (manually) set to "Challenge Mode". This I read on Discord, but I haven't yet looked into it.

King Mechagon DOES have a statistic for mythic difficulty, at least: https://www.wowhead.com/achievement=13620/king-mechagon-kills-mythic-operation-mechagon

This still won't prevent invalid attempts being counted if running the dungeon multiple times while being locked out, but that's just something we have to live with. It also applies to Freehold, Underrot, and King's Rest.