If downstream is constrained to an 8-bit profile, caps queries would still allow I420_10LE as input. If upstream actually sends such a caps event, downstream would fail to accept the high-10 profile.
The following pipeline now fails earlier, during the negotiation phase instead of the stream start:
gst-launch-1.0 videotestsrc ! video/x-raw,format=I420_10LE \
! x264enc ! video/x-h264,profile=constrained-baseline \
! fakesink