Two devices in one Alexa endpoint group

I’m trying to make an Alexa group with a physical MQTT Sonoff-tasmota switch (ON/OFF) and a Milight lightbulb (brightness, color, etc.). Alexa recognises the group in the app but says “The device is unresponsive”. The brightness and color work through the Alexa app but the ON/OFF does not. Through Openhab the switch works fine. Is the setup of two different devices in one group supported by the Alexa binding?

My items:

//Endpoint voor Alexa
Group  Windowsill    "Window-sill"                                {alexa="Endpoint.Light"}

//SONOFF S20 04 {alexa="Switchable"}
Switch					Light_GF_Living_Windowsill				"Window-sill ON/OFF"						( Windowsill,gSwitches,gLights_GF_Living,gLights_All,gMilightC_FF,gLights_GF_Living_MCFF,G_PresenceSimulation )												{channel="mqtt:topic:sonoff-s20-04:power",alexa="Switchable"}

//milight bulb Light_GF_Living_Windowsill
Dimmer					Light_GF_Living_Windowsill_Brightness	"Window-sill"									(Windowsill,gMilight,gLights_All,gLights_GF_Living_Dimmer)																						{ milight="firstfloor;9;brightness",alexa="Lighting" }
Dimmer					Light_GF_Living_Windowsill_ColorTemp	"Temperature"								(Windowsill,gMilight,gLights_All)																															{ milight="firstfloor;9;colorTemperature",alexa="ColorTemperature.colorTemperatureInKelvin" }
Color					Light_GF_Living_Windowsill_ColorPick	"Window-sill Color"							(Windowsill,gLights_GF_Color,gMilight,gLights_All)																											{ milight="firstfloor;9;rgb",alexa="BrightnessController.brightness,PowerController.powerState,ColorController.color" }
Dimmer					Light_GF_Living_Windowsill_ModeSpeed	"Mode speed"								(Windowsill,gMilight,gLights_All)																															{ milight="firstfloor;9;discoSpeed",alexa="RangeController.rangeValue"  }
Switch					Light_GF_Living_Windowsill_WhiteMode	"White mode"								(Windowsill,gMilight,gLights_All)																															{ milight="firstfloor;9;whiteMode",autoupdate="false",alexa="ToggleController.toggleState" }
Switch					Light_GF_Living_Windowsill_NightMode	"Night mode"								(Windowsill,gMilight,gLights_All)																															{ milight="firstfloor;9;nightMode",autoupdate="false",alexa="ToggleController.toggleState" }

I also tried the following, with the same (unsuccessful) result:

//
// Window-sill
//

//Endpoint voor Alexa
Group  Windowsill    "Window-sill"                                {alexa="Endpoint.Light"}

//SONOFF S20 04 {alexa="Switchable"}
Switch					Light_GF_Living_Windowsill				"Window-sill ON/OFF"						( Windowsill,gSwitches,gLights_GF_Living,gLights_All,gMilightC_FF,gLights_GF_Living_MCFF,G_PresenceSimulation )												{channel="mqtt:topic:sonoff-s20-04:power",alexa="PowerController.powerState" [category="LIGHT"]}

//milight bulb Light_GF_Living_Windowsill
//Switch Light_GF_Living_Windowsill "Window-sill ON/OFF" (gSwitches,gLights_GF_Living,gLights_All,gMilightC_FF,gLights_GF_Living_MCFF,G_PresenceSimulation) { milight="firstfloor;9" }
Dimmer					Light_GF_Living_Windowsill_Brightness	"Window-sill"									(Windowsill,gMilight,gLights_All,gLights_GF_Living_Dimmer)																						{ milight="firstfloor;9;brightness",alexa="BrightnessController.brightness" [category="LIGHT"] }
Dimmer					Light_GF_Living_Windowsill_ColorTemp	"Temperature"								(Windowsill,gMilight,gLights_All)																															{ milight="firstfloor;9;colorTemperature",alexa="ColorTemperature.colorTemperatureInKelvin" }
Color					Light_GF_Living_Windowsill_ColorPick	"Window-sill Color"							(Windowsill,gLights_GF_Color,gMilight,gLights_All)																											{ milight="firstfloor;9;rgb",alexa="BrightnessController.brightness,PowerController.powerState,ColorController.color" }
Dimmer					Light_GF_Living_Windowsill_ModeSpeed	"Mode speed"								(Windowsill,gMilight,gLights_All)																															{ milight="firstfloor;9;discoSpeed",alexa="RangeController.rangeValue"  }
Switch					Light_GF_Living_Windowsill_WhiteMode	"White mode"								(Windowsill,gMilight,gLights_All)																															{ milight="firstfloor;9;whiteMode",autoupdate="false",alexa="ToggleController.toggleState" }
Switch					Light_GF_Living_Windowsill_NightMode	"Night mode"								(Windowsill,gMilight,gLights_All)																															{ milight="firstfloor;9;nightMode",autoupdate="false",alexa="ToggleController.toggleState" }

Have you gone through the troubleshooting guide? Most likely, one or more of the items you listed don’t have a proper state.