compositor: significant color issues with "special" resizing parameters

Consider this example:

gst-launch-1.0 compositor name=comp background=transparent \
    sink_1::width=906 sink_1::height=509 sink_1::xpos=36 sink_1::ypos=285 \
    sink_2::width=906 sink_2::height=508 sink_2::xpos=978 sink_2::ypos=285 ! \
        videoconvert ! video/x-raw,width=1920,height=1080 ! autovideosink \
    videotestsrc pattern=white ! video/x-raw,width=1920,height=1080,format=ARGB ! comp. \
    videotestsrc pattern=white ! video/x-raw,width=1920,height=1080,format=I420 ! comp. \
    videotestsrc pattern=white ! video/x-raw,width=360,height=202,format=I420 ! comp.

Expectation here is to get completely white image, since we draw 2 white rectangles on top of white background. In reality though left rectangle is a bit grey-ish and right while similarly gray-ish has red line on top of it:

Something isn't right for sure. In understand that compositor doesn't have anti-aliasing support, but this is really severe artifacting.

Happens on 1.18 stable and happened to me on master long before that.