If trying to combine two identical piece of undamaged armour with a maximum durability of 2147483647 in a grindstone, the result will not appear to be a single piece of armour with a full durability. Instead, the durability of the resultant armour will be 21474834, which also means that the grindstone fails to add extra 5% durability to the repaired armour.

HOW TO REPRODUCE

1. Use the command "/give @s netherite_boots[max_damage=2147483647] 2" to gain 2 same boots;
2. Press down F3 + H and then get a grindstone; 
3. Put the two boots into the GUI of the grindstone, then ensure that your cursor is on the top of the resultant item.