Just a follow-up to my post in case it helps others. I can confirm that this glitch DOES impact the 100% at the end. :-(
I am missing 1 mithril brick and it is from the Tracking Hobbits level where it says I have 0/3 treasures, even though I have confirmed they are in my Treasure Trove.
What I have tried so far:
1. Replayed the level on Free Play digging up all 3 spots where the treasure was. In each case, it gave me a blue stud instead of the treasure, so the game knows I have them. I played until I got the 3rd treasure, and then saved and exited.
My next attempts:
1. Play the whole level on Story mode to see if I can reset the counter.
2. Play the whole level on Free Play, all the way to the end to see if that makes a difference.
I am not hopeful that either will work, so I seem to be stuck. I am disappointed, b/c 4 achievements are stuck due to this 1 glitch.
If anyone else has run across some similar and knows a work-around, PLEASE let me know. :-)