Setting ffmpeg timebase to 1 nanosecond breaks some encoders
!3634 (merged) broke the avenc_mpeg2video encoder, and maybe others. Internally that encoder expects the timebase to be the inverse of the framerate.
After !3634 (merged), this line (that works in 1.20.5) is broken:
gst-launch-1.0 videotestsrc ! videoconvert ! avenc_mpeg2video ! fakesink