..and those horizontal bars exists only with softhddevice? Have you tried other vaapi mediaplayers with vpp enabled on your system? I still think this is due to a incompatible library version or buggy driver.
Not that easy: kodi and mpv use vaapi-vpp, but only for deinterlacing, not the other filters such as the color_balance filters.
Both (compiled) programs work perfect.
The only user of other vpp filters I could found was
after compiling this, I saw the horizontal bars !
and only if setting any vaapipostproc filter.
So you are right, it must be somethng in my system.