time goes by… then AE support wakes up… can’t figure it out
OK I play some more… then we notice this odd non-interlaced message in the project window when we select the PNG RGBA in Interp Footage Lower Field… so I rerender the PNG16 as TIFF16 RGBA with fields and replaced the PNG footage with that and now it works
so I refiled to adobe as bug with PNG format with fields when you use Frame Blending in render settings in a project over 8 bits…
for now workaround is use TIFF16
Pierre