What happens if you select AI mask and manually click the sky area?
Thanks, Andy! Nothing happens.
However, I found that pretty random subject selection modes work – in this case, it did a perfect selection of the foreground only when I selected “flower”. I was then able to invert it to create a sky mask. It also refuses to recognize this enormous structure (photo below) in the foreground, as a possible subject – but was glad to select it – nearly perfectly – if I agreed to call it a “vehicle” 
. It only took moments longer per image to work around its intransigence – once I knew to look for creative categorizations.
It seems to me DxO might want to relax the classification thresholds and just let the software nominate several things as the possible intended ‘subject’ or ‘background’ or ‘sky’. We could then select our intention for it, from the several possibilities. Maybe there are technical reasons that’s not implemented.
