OK, the culprit appears to be this:
(<unknown>:62478): GStreamer-WARNING **: 21:03:20.166: Trying to set string on taglist field 'extended-comment', but string is not valid UTF-8. Please file a bug.
I used ffmpeg to remove metadata from the WebM video asset (with no code changes) and now seeking back to the start of the file is working as expected. Consider a bug filed. 
(Actually, I’m happy to formally do so, I just need to research how that’s done for this project.)
Sorry for talking to myself in this thread, but hopefully this will help someone else who encounters this problem, and/or nudge the appropriate GST developer to fix the underlying problem.
Thanks!