vp9dec: gst-play-1.0 plays video (attached) created with "ffmpeg -f x11grab -i "$DISPLAY" /o.webm" wrong

Steps to reproduce:

• I use debian sid amd64 created today (in fact, it is chroot environment in older debian, but I hope this doesn't matter for this bug report)
• dpkg reports that gstreamer1.0-plugins-good 1.16.0-3 is installed
• ffmpeg reports its version as ffmpeg version 4.1.4-1+b2
• Capture X11 video using ffmpeg -f x11grab -i "$DISPLAY" /o.webm
• Resulting video is attached: o
• Play it with gst-play-1.0 /o.webm
• You will see that colors are wrong. Video looks pink instead of correct white

Notes:

• If I play video using mpv, there is no bug
• When I see this bug report using my browser (Chromium), the browser shows attached video correctly, i. e. I see correct white color, not buggy pink
• If I record video using ffmpeg -f x11grab -i "$DISPLAY" -c:v vp8 /o.webm (i. e. using vp8 instead of default vp9), there is no bug
• If I add -pix_fmt yuv420p or -pix_fmt yuv444p to ffmpeg command line, there is no bug