Salem

I made 2 gates and stupidly didn't use the same key on each lock. To try and tidy things up, whilst making a friend a slave copy of keys, I tried to re-attune one gate and lock. The master key, for that gate, went poof from my inventory, as did the associated slave key. Now it won't let me put a new lock on it as it requires the old master key.

How can this be fixed? Must I destroy the gate and build a new one?

This sounds like a bug to me. For the moment you'll have to nuke the gate though. I'll move this to the bugs thread.

Okay my pal went to work, when he came home and logged in, he found both the slave and master in his inventory. Both keys had disappeared when attuning to him. So all is well.

To my knowledge, keys are still not supposed to jump inventories even during attuning, unless the system was recently changed.