When using vaapih264enc, the VUI descriptor in the resulting stream may be invalid depending on the input image resolution.

Let's consider the following command line:
$ gst-launch-1.0 videotestsrc num-buffers=60 ! video/x-raw, width=1280, height=720 ! vaapipostproc ! vaapih264enc ! mp4mux ! filesink location=test.mp4

Then, if we use ffprobe on the resulting mp4 file:
$ ffprobe test.mp4

We get the error "[h264 @ 0x12e3d40] Overread VUI by 8 bits"

However, with:
$ gst-launch-1.0 videotestsrc num-buffers=60 ! video/x-raw, width=640, height=360 ! vaapipostproc ! vaapih264enc ! mp4mux ! filesink location=test.mp4

We only get a warning from ffprobe: "[h264 @ 0x1388d40] Truncated VUI"

And finally, if we go full 1080p:
gst-launch-1.0 videotestsrc num-buffers=60 ! video/x-raw, width=1920, height=1080 ! vaapipostproc ! vaapih264enc ! mp4mux ! filesink location=test.mp4

We get no error or warning.

I've tried encoding videos using x264enc in comparison, I get no such issue.

While many video players seem to tolerate an invalid VUI descriptor, it's worth noting that others don't (like the Safari video player on iPad).