When I had one good key and one slightly iffy one (dead battery, worn casing but would still start car) I bought a new one and got it programmed. That knocked out the iffy one and that wouldn't even pull into the socket. Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.

mmmmm, there are a limited number of keys/remotes available for these Merc derived units and it makes sense to disable 1 if a new one is issued. I think the limit is 10 though. Offset your Carbon emissions by boycotting Baked Beans and all Green Vegitables.