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#0  __memset_avx2_unaligned_erms () at ../sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S:192
#1  0x00007fe1128faff1 in memset (__len=640, __ch=0, __dest=<optimized out>) at /usr/include/x86_64-linux-gnu/bits/string_fortified.h:71
#2  _draw_background (composite=<synthetic pointer>, y_end=<optimized out>, y_start=360, outframe=0x7fdfd0bc91d0, comp=0x7fe040088590 [GstCompositor|compositor_1]) at ../gst/compositor/compositor.c:1155
#3  blend_pads (comp=0x7fdfd0bc9130) at ../gst/compositor/compositor.c:1175
#4  0x00007fe1128fb187 in gst_parallelized_task_thread_func (data=0x7fdfc4004eb8) at ../gst/compositor/compositor.c:901
#5  0x00007fe1149a81b1 in  () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#6  0x00007fe1147ff590 in start_thread (arg=0x7fdfc3ae9640) at pthread_create.c:463
#7  0x00007fe1145d4223 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Pipeline is similar in its mechanics to this:

gst-launch-1.0 videotestsrc is-live=true ! video/x-raw,width=1280,height=720,format=I420 ! \
    compositor background=transparent ! \
    compositor background=black ! video/x-raw,format=BGRA ! \
    videoconvert ! video/x-raw,framerate=30/1,format=I420 ! \
    fakesink

However, I can't reproduce it with gst-launch for some reason.

Changing background=transparent to background=black on the first compositor fixes the issue.