I wouldn't really consider this a "bug," though I understand it may not be the behavior you expect. All group devices get assiged one specific device "type" or driver. For all group devices, this looks more or less like an RGBW bulb, which contains the widest set of capabilities you might find in an individual device (switch on/off, level/brightness, color temperature, RGB color, etc.)., regardless of the individual capabilities of the member devices.
Notably for this situation: it does not explicitly declare itself as a shade. However, unless your devices are particularly odd, everything you need should be there: the "Open" and "Close" commands are (almost always?) the same as the "On" and "Off" commands (available on switches). Similarly, the "Set Position" command (shades) can be done with "Set Level" (dimmers) instead. For looking at or comparing attributes, the "position" (shades) attribute should correspond to the "level" (dimmer) attribute.
So: the solution is to pretend you're adjusting a dimmer instead of a shade. RM has actions that can handle all of that. Alternatively, you could adjust a "blind" but choose them all individually instead of the group. I'd probably do the former. If your devices are odd and don't implement the dimmer-type commands/attributes, then I suppose you'd need to, though all drivers I've run into (by far not all...but this is probably something they all do to make themselves more generally useful in a wider variety of apps) do this, so you probably don't.
I guess its more of an improvement than a bug, Hubitat is very technical and requires a lot of mental gymnastics, "this group of shades is now a dimmer!" for it to become a serious contender for the average person the number of times you have to overlook weirdness like that has to be seriously reduced for people to actually want to use the product without frustration and confusion.