https://wccftech.com/new-2018-ipad-pro-camera-features-smart-hdr-but-no-portrait-mode-on-rear-camera/

This is somewhat of a bummer: the new 2018 iPad Pro tablets can’t do portrait mode using the rear camera. The feature works on the front facing camera only.

If you’ve been wondering whether or not the new iPad Pro features Portrait mode, then the answer is a yes and a no at the same time. Yes, because it has a TrueDepth sensor for Face ID enabling Portrait shots. No, because Portrait mode is exclusive to the front facing camera only not the rear one.