If the spring block config is changed, the spring block remains registered to the block registry on the server, causing a mismatch when a client without the mod connects, thus being incompatible with vanilla clients.

You are probably using the forge version aren't you?
Because I tested this on fabric and it works pretty well but forge might handle things differently.
I will have to look into that.

I tested this and it seems like vanilla clients can connect without a problem to forge or fabric servers running this mod.
It might be that only forge clients that are missing the mod are complaining.

Sorry, i should've been more specific - yeah, I've only tested on a forge client without the mod.