Room Lights supports Zigbee Group Messaging (ZGM). This allows multiple Zigbee lights to activated by a single command on the Zigbee network. Doing so both cuts down on network traffic, but also causes the lights to respond all at the same instant.
However, there are limitations to this feature, and understanding exactly how it works sheds light on these. ZGM is tied to an Activator Device. When the Activator device is commanded, all of the lights in the ZGM group are commanded in exactly the same manner. For instance, if the Activator Device is turned on, the ZGM lights are also turned on. If the Activator Device has its color temperature set, then the ZGM lights have their color temperature set, etc.
There are a couple of things to note: These devices commanded according to how the Activator Device is commanded comprise a group. Such a group may also include non-Zigbee devices, and these are activated one at a time just as the ZGM devices are activated all at once. It is only in this context of a group of devices that ZGM makes sense. There is no other context where ZGM will work. Specifically, when there are different settings to be established for the various devices (a scene), ZGM won't make sense as ZGM causes the exact same command to be sent to all of the commanded devices.
There was a mistake made in the initial implementation of Room Lights with respect to when ZGM is enabled. Specifically, ZGM was treated as an opt-out option, and whenever there was both an Activation Device and multiple Zigbee devices, ZGM was set up. This would cause Scenes to not work as expected, since ZGM doesn't support different activation values. Consequently, to correct this mistake, ZGM has been converted to an opt-in option. When a Group-2.1 app is imported into Room Lighting, ZGM will be used if it was selected in the Group-2.1 app, and not otherwise. When a Scene-1.2 app is imported to Room Lighting, ZGM will not be enabled.
Notice: If you have been using ZGM intentionally, you must open the Room Lights app after updating to 2.3.3 and select the new option to affirmatively choose ZGM. ZGM will be deselected when the app is opened. If you never intended to used ZGM and had a scene not activated as expected, this might be the cause. In this case, simply opening the Room Lights app will disable ZGM.