Using addon version r745, I have just killed Yogg-Saron twice with Alone in the Darkness using a priest and rogue. Neither kill gave me credit for Mimiron's Head and I had to manually increase my kill count. I know that I met the conditions for looting the mount because I got the Alone in the Darkness achievement but no credit on the addon.

Kill statistics aren't updating. Unless Blizzard fixes them or a robust workaround is added to Rarity, attempts won't register.
See #670 and various other mentions of this problem here, on WowAce, or Discord for details. Closing as duplicate :)

FWIW, you can locally add a custom item (copying the same settings, except for the statistic ID) with BOSS or NPC as the method of detection. Albeit not a general solution, it should technically work and give you a working attempt count in the meantime.