Yep. As soon as I updated all iOS devices to 14.2 today I started getting the “No Members” error when using any of my groups in Mail.
The Groups, populated correctly are still there in the Contacts app itself.
For a long time I have had trouble using Group contacts in the BCC field. Nothing happened, without any error thrown even though they worked in the “To” field. My Workaround: I would have to drag and drop each contact from the To field to the BCC field.
Now nothing in any field works and they all say “No Members.”
This problem is identical on two different iPad models, but my IPhone SE is fine. Go figure!