When you use an anvil to rename an item, the item receives a RepairCost tag even when it cannot be enchanted or repaired. Upon reverting the custom name on an anvil, the item retains the RepairCost tag, preventing otherwise stackable items from stacking with other non-renamed items of the same type.

How to reproduce:

1. Rename a stackable item, e.g. an Egg, on an anvil, e.g. to "Renamed".
2. It is now NBT tagged with:
   1. RepairCost: 0
   2. {display: {Name: '{"text":"Renamed"}'}

1. Put the renamed item onto an anvil and delete its custom name, reverting its name to be "Egg".
   • Note: You will encounter MC-149761 when doing this.
2. It retains the RepairCost tag, preventing it from stacking with other Egg items.

For many items there exist workarounds: Blocks can be placed and destroyed, dropping a non-tagged item, and some items can be crafted into other forms from which the item can be re-crafted (e.g. iron -> iron block -> iron). Still it is a nuisance.