That's great feedback. Funny that when I let my friend test the game, he also found the weapon chaining bug and I though it was kinda cool so I kept it in. When you buy weapon/mask and then take the worse one, it unfortunately takes the worse one (although the masks are discounted and the weapons are free) and to open the door you need three keys.
(Spoiler: 1 in world, 1 in shop, 1 from the boss)