With the release of Siege of Paris, I obtained a few additional armor pieces. I went to upgrade everything to mythical only to find that the Pants/Leggings category no longer appears now. I don't know if this is occurring because there are too many armor pieces in the menu selection now but I am unable to upgrade my new armor pieces all the way to receive the full benefits of mythical status. At this point it affects ALL pants gear at this point; base game; Wrath of the Druids, and Siege of Paris because nothing can be enhanced or downgraded in the Pants category.
Here is a video showing at Gunner in Ravensthorpe (also occurs at the Blacksmith in Ireland), that the Pants/Leggings do NOT appear in the Enhance Gear or Downgrade Gear sections:
There is a workaround though. Let's say you have three "Pants" items you want to upgrade from Flawless to Mythical. The workaround is to equip one of them, open the Blacksmith, and Enhance the gear from the "Currently Equipped" section, then exit the Blacksmith, equip the next one, and repeat.