armor enchantments dont work with resistance

i have maxed out protection 4 diamond armor and when i took damage from a mob compared to no protection i take the same damage (ONLY WITH RESISTANCE)

how to reproduce

1. get any armor
2. take damage from a mob
3. check your stats
4. it'll say that much damage taken
5. now enchant with protection or so
6. take damage from a mob again
7. check your stats
8. when you divide by 2 the first result is the same

similarly to feather falling:

1. get any armor
2. take fall damage
3. check your stats
4. it'll say that much damage taken
5. now enchant your boots with feather falling, you can apply protection if you want
6. take fall damage again
7. check your stats
8. when you divide by 2 the first result is the same