Level 0 is a special case. 0 = switch off. How the driver dimmer level changes or does not when the switch is set to off can be based on either the device driver or the device itself. You have to know how each device works to make any sense of expectations. None of my dimmers (Lutron and Z-Wave) change the driver level to 0 when setting the level to 0, they just turn the switch off. I do see that a vanilla Virtual Dimmer does change the driver level to 0.
Also, just a note. The Groups app you are using is generally fine, but it was superseded in 2022 by Room Lighting. There's not a ton of difference and it may not (or may) be worth changing. Here is some documentation on the group functionality in RL.
