Using a key panel on chests locked with the Locks mod. (https://www.curseforge.com/minecraft/mc-mods/locks)
will cause the chest to be "destroyed" and dropping items on the ground.

Then it will ask to set up a password. By setting it and opening the chest there will be a new loot table of items generated inside, basically duping items.

This is not an issue with the Locks mod. Placing a chest down, locking it, then converting it to a password-protected chest does not spill out the contained items. This issue happens with any chest whose loot has not been generated yet.