I'm adding a number of containers to my mod, I set them to "Needs a Key" and assign keys to them, sounds like it should work fine, but it doesn't! The containers show up in-game, but they're not showing as locked! I tried all lock levels and nothing works! If anybody can help me, I would be grateful!

If they're owned by the player, or located in a cell owned by the player, they will not be locked to the player. However, they will be locked to NPCs.