vaapipostproc: src output RGBA when specify format=argb

$ gst-launch-1.0 -vf videotestsrc num-buffers=1 \
  ! video/x-raw,format=NV12,width=720,height=480 \
  ! vaapipostproc format=argb \
  ! filesink location=out-vpp-ARGB.rgb

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
Got context from element 'vaapipostproc0': gst.gl.GLDisplay=context, gst.gl.GLDisplay=(GstGLDisplay)"\(GstGLDisplayX11\)\ gldisplayx11-0";
Got context from element 'vaapipostproc0': gst.vaapi.Display=context, gst.vaapi.Display=(GstVaapiDisplay)"\(GstVaapiDisplayGLX\)\ vaapidisplayglx0";
/GstPipeline:pipeline0/GstVideoTestSrc:videotestsrc0.GstPad:src: caps = video/x-raw, format=(string)NV12, width=(int)720, height=(int)480, framerate=(fraction)30/1, multiview-mode=(string)mono, interlace-mode=(string)progressive, pixel-aspect-ratio=(fraction)1/1
/GstPipeline:pipeline0/GstCapsFilter:capsfilter0.GstPad:src: caps = video/x-raw, format=(string)NV12, width=(int)720, height=(int)480, framerate=(fraction)30/1, multiview-mode=(string)mono, interlace-mode=(string)progressive, pixel-aspect-ratio=(fraction)1/1
/GstPipeline:pipeline0/GstVaapiPostproc:vaapipostproc0.GstPad:src: caps = video/x-raw, format=(string)RGBA, width=(int)720, height=(int)480, framerate=(fraction)30/1, interlace-mode=(string)progressive, pixel-aspect-ratio=(fraction)1/1, multiview-mode=(string)mono, multiview-flags=(GstVideoMultiviewFlagsSet)0:ffffffff:/right-view-first/left-flipped/left-flopped/right-flipped/right-flopped/half-aspect/mixed-mono, colorimetry=(string)sRGB
/GstPipeline:pipeline0/GstFileSink:filesink0.GstPad:sink: caps = video/x-raw, format=(string)RGBA, width=(int)720, height=(int)480, framerate=(fraction)30/1, interlace-mode=(string)progressive, pixel-aspect-ratio=(fraction)1/1, multiview-mode=(string)mono, multiview-flags=(GstVideoMultiviewFlagsSet)0:ffffffff:/right-view-first/left-flipped/left-flopped/right-flipped/right-flopped/half-aspect/mixed-mono, colorimetry=(string)sRGB
/GstPipeline:pipeline0/GstVaapiPostproc:vaapipostproc0.GstPad:src: caps = video/x-raw, format=(string)RGBA, width=(int)720, height=(int)480, framerate=(fraction)30/1, interlace-mode=(string)progressive, pixel-aspect-ratio=(fraction)1/1, multiview-mode=(string)mono, multiview-flags=(GstVideoMultiviewFlagsSet)0:ffffffff:/right-view-first/left-flipped/left-flopped/right-flipped/right-flopped/half-aspect/mixed-mono, colorimetry=(string)sRGB
/GstPipeline:pipeline0/GstFileSink:filesink0.GstPad:sink: caps = video/x-raw, format=(string)RGBA, width=(int)720, height=(int)480, framerate=(fraction)30/1, interlace-mode=(string)progressive, pixel-aspect-ratio=(fraction)1/1, multiview-mode=(string)mono, multiview-flags=(GstVideoMultiviewFlagsSet)0:ffffffff:/right-view-first/left-flipped/left-flopped/right-flipped/right-flopped/half-aspect/mixed-mono, colorimetry=(string)sRGB
/GstPipeline:pipeline0/GstVaapiPostproc:vaapipostproc0.GstPad:sink: caps = video/x-raw, format=(string)NV12, width=(int)720, height=(int)480, framerate=(fraction)30/1, multiview-mode=(string)mono, interlace-mode=(string)progressive, pixel-aspect-ratio=(fraction)1/1
/GstPipeline:pipeline0/GstCapsFilter:capsfilter0.GstPad:sink: caps = video/x-raw, format=(string)NV12, width=(int)720, height=(int)480, framerate=(fraction)30/1, multiview-mode=(string)mono, interlace-mode=(string)progressive, pixel-aspect-ratio=(fraction)1/1
Pipeline is PREROLLED ...
Setting pipeline to PLAYING ...
New clock: GstSystemClock
Got EOS from element "pipeline0".
Execution ended after 0:00:00.000477780
Setting pipeline to PAUSED ...
Setting pipeline to READY ...
Setting pipeline to NULL ...
Freeing pipeline ...

If we use videoconvert instead of vaapipostproc, the output is ARGB as expected.