It’s a zigbee controller, and the Xioami outlets are also zigbee end devices (just like bulbs).
Xioami only expects their devices to be used with their own gateway, and they don’t always perform as expected in a SmartThings system.
It may be necessary to
- Remove the dimmer
- Confirm all the outlets are still working OK with ST
- Unplug all the outlets
- then add the dimmer
- Then plug the outlets back in.
That should keep the outlets from adding themselves to the dimmer’s control group when the dimmer is added to the network.
I know it’s a lot of extra work, but that’s the hidden cost of using the inexpensive Xioami devices. 