A group only has 2 attributes. A list of entity ids and a friendly name if you gave it one.
An attribute for the whole group makes no sense as each thing in the group can be completely different and in a different state. And often, they will have overlapping attributes that conflict with each other.
Did you create a light group? Or a regular group?
Actually, I’m not sure it makes a difference. A light group does a few extra things with attributes, but I don’t think it exposes them to the attributes list in the group. It uses them to determine if any light is RGB it would provide a color picker, etc.
Light group must be defined in configuration.yaml. group.yaml it’s for regular groups only. Yes… it sucks.
BTW Light group is not the same as group. Someone has explained it a few weeks ago.