I used to drag my .ipa file into iTunes and sync to test my ad-hoc build. The 'new' iTunes (12.7.0.166) doesn't contain apps any more. So how are you getting your .ipa onto your physical device?
I have updated an ipa onto my iPhone this morning in iTunes 12.7 on Mac.
To get the ipa loaded I dragged and dropped the ipa file onto the 'On My Device' panel in the left hand column of iTunes. The status bar indicated that it was syncing the information, and the app loaded correctly.
Hope this helps.