compositor: image is visible under other non-transparent image

Here is a simple reproducible example:

gst-launch-1.0 \
   videotestsrc ! compositor name=comp ! videoconvert ! autovideosink \
   videotestsrc pattern=solid-color foreground-color=4279769112 ! video/x-raw, format=RGBA ! comp.

Expected behavior is that first non-transparent videotestsrc will be covered completely with solid grey color, but it is very easy to see that there is still a significant pattern behind grey color.

I think this is a significant bug in the way compositor works. Removing RGBA format fixes it.

Reproducible both on 1.16.2 and 1.17.2 (GIT)