Force libavfilter to use a format different from yuv420p for the input to the vflip filter:
noformat=pix_fmts=yuv420p,vflip
Convert the input video to any of the formats not contained in the list:
noformat=yuv420p|yuv444p|yuv410p
noformat=pix_fmts=yuv420p,vflip
Convert the input video to any of the formats not contained in the list:
noformat=yuv420p|yuv444p|yuv410p