User is not the owner of the realm but has operator permissions.

When using an Anvil to rename an item, it appears to work properly and displays the chosen name in the Anvil UI.  When returned to inventory/hot bar, the name appears as hashes (####).  The number of hashes equals the number of characters in the chosen name.  Have tested on different items: shulker box, netherrack and diamond pickaxe. Repeating the rename process produces the same results.