Realms worlds do not allow renaming of items. When name tags are placed in the anvil and the name is changed the anvil shows an 'X' across the arrow to indicate that the operation is invalid.  Additionally renaming items while combining (tested with enchanted book) does not show a change in the experience cost, the operation is allowed but the name is not retained on the combined item.  World once downloaded does allow the anvil to work as expected issue is only present on realms