GStreamer Pipeline Works But After Some Seconds fdsink Gives "Ilegal Seek" Error

[Elliot Killick]

Here is a simpler PoC that gives the same error after waiting for some
seconds if anybody has got some ideas:
$ gst-launch-1.0 -q videotestsrc ! avimux ! fdsink | gst-launch-1.0 -q
fdsrc ! avidemux ! autovideosink

Same error shows up after letting the video play successfully for a bit:
ERROR: from element /GstPipeline:pipeline0/GstFdSink:fdsink0: Could not
perform seek on resource.
Additional debug info:
gstfdsink.c(592): gst_fd_sink_event ():
/GstPipeline:pipeline0/GstFdSink:fdsink0:
Error while seeking on file descriptor 1: Illegal seek
ERROR: from element /GstPipeline:pipeline0/GstFdSink:fdsink0: Could not
perform seek on resource.
Additional debug info:
gstfdsink.c(592): gst_fd_sink_event ():
/GstPipeline:pipeline0/GstFdSink:fdsink0:
Error while seeking on file descriptor 1: Illegal seek
ERROR: from element
/GstPipeline:pipeline0/GstVideoTestSrc:videotestsrc0: Internal data
stream error.
Additional debug info:
gstbasesrc.c(3072): gst_base_src_loop ():
/GstPipeline:pipeline0/GstVideoTestSrc:videotestsrc0:
streaming stopped, reason error (-5)

My GStreamer is up-to-date, here are the version strings:
gst-launch-1.0 version 1.18.2
GStreamer 1.16.2

Can anyone reproduce this behaviour? If so, please let me know so
perhaps we can get to the bottom of this.

Thanks,
Elliot