OMG - I get the OP's issue.
From: Wireguard not working, OpenVPN protocol works - #6 by lleachii
[I think] The OP is also using this box - thinking it's used for another purpose. I agree with the OP - but will have to take some time to better re-state the "bug". I'll be honest, I'm trying to recreate a use case that leads me to saving the config. Wireguard implies the key is transferred "out-of-band".
EDIT:
When I say "saving config" - I specifically mean UCI (as you're making a config for another device - i.e. the mobile Wireguard client). Basically, I think the fields are just a generator, you're not to then save the config - I'm testing it.