Got a response from @ebaauw on the initial request:
See the latest DDFs. I think the old (round) wireless dimmer is the only IKEA controller that still sends group casts (probably because its firmware hasn’t been updated recently). The others accept the binding allright, but don’t send group casts. They should not expose config.group. and no associated group should be created on pairing, but it could be that the legacy code interferes. Not sure about existing resources, you probably need to remove and re-pair the device.