Currently, when you have a group layer, the legend aggregates all layers within the group layer in the parent group layer legend option of the map layers widget. The individual map layers widget remains grayed out. This applies to as of the February 2025 AGOL update to EB. In WAB, this request is supported as legends for each layer within the group layer can be viewed.
EB (Legend is displayed in parent grouping instead of each individual layer):

WAB (legend is displayed for reach individual layer, regardless of grouping):
