Follower's items disappear after "The Final Descent" quest

Question

Before I played said quest, I told my follower to wait outside Raven Rock Mine and then I entered. I completed the quest and read the Black Book. When I was at the point where I could choose a Black Book perk, I chose the one that makes me do no damage to my follower. When I exited the Black Book, I came back to the point where I first read it. Surprisingly, my follower, Illia, was standing there as well, but with none of the armor/items I gave her, only her standard robes and spells.

I could reload from where I left her outside the Mine but I think the same thing will happen after I do the quest. Also, checking the wiki, I found this bug: "Her inventory is known to reset."

If anyone knows a way of completing the quest without her inventory disappearing, that would be great (because the Bloodskal blade is awesome!).

Answer 1

Sounds like a simple solution (because you still have a good save file):

• Reload the save from before the quest
• Remove all important items from her inventory (probably everything)
• Do quest
• Return said inventory to follower

Also, if you are on pc you can download The Unofficial Skyrim Legendary Edition Patch.
This is a very reliable mod that fixes a lot of bugs in the vanilla version of Skyrim. And according to the change log here, one of the fixes is Illia's inventory reset bug.
Just use ctrl+f to search for "Illia" in the change log and you will find it.

Actor Fixes

• Illia has been given a static spell list and flagged as unique to prevent an issue where her inventory would reset after she had been
  left alone for enough time. She's available as a follower and this
  would result in any items given to her by the player being lost when
  she resets. Also removed the leveled dagger from her inventory since
  she was receiving an identical leveled dagger in her alias in
  dunDarklightQST.

This also answers why you are losing the items in her inventory, you are leaving her alone for to long.