I can't see if I'm doing anything wrong, or if this is expected behavior (and Field Maps is not at parody with Map Viewer and Experience Builder).
I have set a group layer to use Exclusive Visibility in the Map Viewer, and it works as desired.

It also works as expected in Experience Builder (left) but not in Field Maps (right). It seems that the layers in Field Maps are still functioning as independently visible, and the bottom one is on by default (when it should be the top one as shown above and in ExB).

P.S. Yes, I am using the latest version of the app and it's on an Android Galaxy Tablet and Phone.
Lindsay Raabe
GIS Officer
Forest Products Commission WA