I have enchanted an efficiency VI, Unbreaking III, Fortune III Diamond pickaxe.
After Testing the pickaxe on multiple sources, including Coal, Restone, Lapis, Diamond, Quartz, Nether gold ore and Emerald; Only one of each ore drops, unlike the supposed 3 or 4.
This isnt a luck scenario, as I've tested it multiple times on large ore veins to no avail.
This bug removes the functionality of fortune, and makes the enchantment entirely useless.