I have done some testing of this bug. It looks to me like the hardware advanced and fast deinterlace may have stopped working with the release of the RPI3. I am unable to see advanced deinterlace working at all on RPI3. Earlier versions of raspbian will not boot on the RPI3.
With a small test program and lots of playing around I have found that advanced deinterlace only works with a tunneled path from decoder to deinterlacer. MythTV normally has the deinterlace as part of the video output stage.
There was a firmware change with firmware version 2d0f7ec (July 8, 2016). The nFlags now carry deinterlace information which must be passed forward with each buffer.
