In Minecraft, items of the same type (like Diamonds or Cobblestone) are expected to stack up to 64. However, the game tracks specific metadata called data. When you rename an item in an anvil, the game adds two pieces of data: display.Name : The new custom name.
Setting up that handle renamed items
MC-125046 is a documented where items renamed using an anvil are assigned a "RepairCost" NBT tag, preventing them from stacking properly with identical items. Technical Overview 125046
: Renamed block entities (like Chests or Shulker Boxes) are also affected, meaning you cannot stack renamed storage containers. In Minecraft, items of the same type (like
The bug occurs because the game applies a RepairCost tag even if the item hasn't been repaired, just renamed. Since the game only stacks items with identical NBT data, a renamed item (with RepairCost: 0 or higher) will not stack with a standard item (which has no tag at all). How to Manage Renamed Items Setting up that handle renamed items MC-125046 is
Even if two items have the same custom name, they might have different RepairCost values if one was used in an anvil more times than the other. 2. Using the Grindstone
Using in Creative Mode to manually fix item tags Minecraft Snapshot 23w33a