A Horde of Hoofbeats not tracking correctly

[Sparx-wyrmrest-accord (Sparx)]

Lord of the reins

Hello so my lord of the reins achieve is at 299/300 i learnt the tame blood feaster mount no achieve check still 299/300 do a full UI reset delete all addons and still nothing when i check my armory its says 300/300 and achieve not awarded https://worldofwarcraft.com/en-us/character/us/khazgoroth/noctisdrache/achievements/collections/mounts (link to armory achieve section) i submitted a ticket and the GM said they cannot do anything other then learn more mounts and submit a bug report

Been happening for a while now. Not entirely sure what causes it, and the only fix is to overcap with several more mounts.