My guess is that this is some unwanted behaviour (you could call it a bug) within the Homey SDK.
The Shelly App will not touch the the dim and/or white level when only the onoff capability is used. There is however a onoff.whitemode subcapability that will set the white to 0 when triggered. My guess is that the flowcard for turning off all zones also switches at least the onofff subcapabilities which is causing this behaviour for the white level. I cant really explain why it would also set the dim to 0 though.
Anyway, as you already guessed yourself. This is not for me to fix but something Athom needs to look at.
