If a stackable item has a 'tag' (such as custom names, lores, or enchantments), it can still be stacked and unstacked normally in the inventory. However, if you drop multiple instances of such an item on the ground, they do not stack.
This disrupts item detectors which rely on stacking mechanics, and thus limits the use of custom items in maps (yes, this is how I ran into the issue - so much for having magical relics that act as keys).
What I expected to happen was...:
The custom items should have joined together into a stack on the ground.
What actually happened was...:
The items did not stack on the ground, as though they were non-stackable.
Steps to Reproduce:
1. Take a stack of any item and rename it on an anvil.
2. Observe that the items can be unstacked and restacked freely in the inventory.
3. Drop the items on the ground. Observe that they do not stack.
4. Drop normal stackable items on the ground. Observe that they join to form a single stack. There may be some delay before this happens.