FFmpeg read, ignoring remaining -vf options: Use ',' to separate filters
It's a false positive in this case. You can ignore it. The check is supposed to catch duplicate filter invocations for the same output stream, e.g. -filter:v:0 scale=hd1080 -filter:v:0 hue=s=0 where only the hue would get applied. The correct way is -filter:v:0 "scale=hd1080,hue=s=0". But the check is buggy.

There's an open ticket for it.