Item Frame, item disappears after removing from frame

single player, survival mode, difficulty normal, savanna plateau, living in structure under a waterfall which has been redirected using cobblestone, stone and stone slabs (these blocks create the ceiling of the structure)

1. Crafted new item frame
2. Crafted Diamond Helmet
3. Enchanted Helmet (unbreaking 1 and aqua affinity)
4. Placed item frame on vertical wall immediately below ceiling
5. Placed Helmet in item frame
6. Changed my mind (did not look good!)
7. Removed helmet from item frame with iron pick axe
8. Helmet leaped into ceiling and disappeared! Also not in hotbar or inventory (five diamonds and experience points gone!)
9. Removed ceiling slabs and blocks searching for the helmet
10. Flooded the place but could not locate the helmet
11. Going back now to fix the flood damage!