Source: Multiple (long-standing issue)

Blizzard broke kill statistics for every raid that's also available on Timewalking difficulty. Because Rarity relies on these statistics tracking just won't work for the items obtained from those raids, unless the method of detection is changed (e.g., to NPC or BOSS). However, because people have been using those stats for years changing the method will almost certainly lead to unintended side effects, such as lost attempts or other inconsistencies that would be extremely difficult to troubleshoot. That's why I haven't done much about the problem.

Recently, a potential workaround has been proposed:

I don’t suppose you could just add a list of bosses (or raids if the data is by any chance organized that way) to ignore blizzard’s kill stats on. Fall back to rarity statistics for them. If the rarity counter is zero and the blizzard counter isn’t, initialize the rarity counter from the blizzard counter, so as to not throw away the kill statistics collected before a boss is added to the blizzard-broken-kill-stats list.

Source: Discord

I still think modifying the method for these items might be risky. But clearly, Blizzard isn't going to fix the problem before the advent of the next ice age. So it might be worth looking into as, presumably, a decent number of people are farming those mounts still?

Affected items (incomplete list):

• Mimiron's Head
• Firelands mounts (and possibly pets? IIRC they're using the NPC method though)
• Any other Timewalking raids? (TBD)