I’ll start off by saying I don’t remember seeing an issue like this before the new 1.18.30 update. When you /replaceitem for any armor slot, the armor does not visually appear. In survival/adventure, the armor icons above health do not appear either. These items do exist on the player but do not appear. When wearing this invisible armor, it still acts normal. You get the armor’s benefits. Note that this only effects players.

Also, when you swap out this “invisible armor” using the ‘interact/use’ button to equip armor, all “invisible armor” goes back to normal, including the ones you didn’t swap. But now a new issue occurs, a certain /replaceitem’ed armor slot you used becomes bugged. You cannot remove it from within your inventory unless you swap that armor piece. Everything then seems to become normal. I say certain because I’m unsure myself.

1 last piece of information is that if you just /replaceitem your armor slots & do nothing else to mess with those armor slots, after X time it goes back to normal. I’m still confused on this one as well.