I have maxed the mount Broom (obtained via Christmas Winter festival event) to max the item level to 2000+. I gave me the skill "rejuvenating favor" which thus had 2000+ item level (mythic mount level). Then later I obtained and linked the Griffin : it erased my maxed skill to give me a 800+ skill level as if the skill was scaled on the Griffin level and no more on the maxed Broom. I only have this bug with this mythic mount (the broom) and the bug happened at this precise moment (when I got a new mount, not maxed, with the same skill) and not when I modified any of my equipment.
Indeed, if i equip another skill than "rejuvenating favor", another skill that comes from an other mythic mount, the item level is 2000+ like expected for a mythic mount.
The power of the skill should be scaled on the highest mount with the same skill that is why it is a bug because it is actualy scaled on my last obtained mount, and not my highest mount.
Thank you, have a nice day