I first discovered this when taking a protection IV netherite helmet to my anvil to add projectile protection II, however the minute I put the helmet in the red X symbol was displayed by the anvil. I attempted to combine multiple items with enchanted books as well as trying to combine enchanted books, and every time I was shown the red X even though multiple enchantments has been a game mechanic for years.  This continued after uninstalling and reinstalling the application (java for mac os).