saby:

I wonder if it has something to do with USB type connection : http://forums.pinnaclesys.com/forums/permalink/777720/777720/ShowThread.aspx#777720

 If that were the case, wouldn't the photos also be invisible to Studio?

Since we are speculating, perhaps it has something to do with the way Apple handles transfer of HEIF files to non-compliant devices.  If iOS sees that the receiving device cannot handle HEIF images it automatically converts them to jpg during the transfer.  I don't know whether it does anything to video.