Can confirm. The number of times that triggers is actually equal to the number of item stack in your inventory (not including offhand slot and armor slots). And this also happens when opening block GUI, not exclusive to non-player entities.
Replacing the armor with the same type of armor but different components still play the equipment sound.
Can confirm in 1.21
It was the hard reload that will shuffle the order, which means the order can be different after each single player world reopen or server restart.
I did more testing. A single block with given namespace only works if the given namespace is "minecraft", otherwise it blocks nothing.
I think I will do another report for this.
Confirmed for 1.19.4
Can confirm in 1.21.2