From Elijah M on discord:
I think I found a bug, or at least I think this is undesired behaviour. When I create a config that is synchronized between the client and server, the config on the dedicated server overrides the config in single-player, with the integrated server. When I set an option in a single-player world, and then leave and join a dedicated server, the GUI now shows the value from the dedicated server, and the value I set in the single-player world does not override it, which is as it should be. However, when I leave the server and join a single-player world again, the setting from the dedicated server has overridden the value I set at first, even if I did not change this on the dedicated server. If there are settings that only influence the server side of a mod, this prevents a server admin from having different settings in single-player worlds than on the server they manage. I know I could use RegisterType.SERVER, but I want the client to be able to edit the config using the GUI.

I probably want to define options this behavior, or consider removing the client saving in certain circumstances.

Fixed in 0.6.8