Ok, so they are definitively not switches ^^ (even they have this name for domoticz), but 100% plugs.
I still can't reproduce bug, but I have only 1 to put in a group.
So I have 1 group just for it with only 1 plug.
In domoticz :
- the plug is a "type : light switch, switch,on/off"
- the group is "type colorswitch RGBWWZ, Dimmer
If I click on the plug I have
2019-06-06 18:46:24.995 (deCONZ) Classic Data : [{'success': {'/lights/10/state/on': True}}]
2019-06-06 18:46:24.995 (deCONZ) ### WebSocket Data : {'r': 'lights', 'id': '10', 't': 'event', 'uniqueid': '7c:b0:3e:aa:00:b0:7b:11-03', 'state': {'alert': None, 'reachable': True, 'on': True}, 'e': 'changed'}
2019-06-06 18:46:24.996 (deCONZ) ### Update device (deCONZ - Plug Osram) : {'nValue': 1, 'sValue': 'on'}
If I click on the group I have
2019-06-06 18:47:36.081 (deCONZ) Classic Data : [{'success': {'/groups/5/action/on': False}}]
2019-06-06 18:47:36.081 (deCONZ) ### WebSocket Data : {'r': 'lights', 'id': '10', 't': 'event', 'uniqueid': '7c:b0:3e:aa:00:b0:7b:11-03', 'state': {'alert': None, 'reachable': True, 'on': False}, 'e': 'changed'}
2019-06-06 18:47:36.082 (deCONZ) ### Update device (deCONZ - Plug Osram) : {'nValue': 0, 'sValue': 'off'}
2019-06-06 18:47:36.087 (deCONZ) ### WebSocket Data : {'t': 'event', 'r': 'groups', 'state': {'all_on': False, 'any_on': False}, 'id': '5', 'e': 'changed'}
2019-06-06 18:47:36.087 (deCONZ) ### Update device (deCONZ - test) : {'nValue': 0, 'sValue': 'off'}
Maybe it's perhaps I m using the last beta version, but your bug is realy strange. If you have time, can you check on domoticz logs (setting/logs) if you have error message in the bugy situation (plug in a group in phoscon)