I think it was that "multipolygon" comment that made the difference!  I compared it with the Fanlew, and the Fanlew said it was a multipolygon while the Plank Road SF did not.  Dunno why, since I created them both the same way.  Anyhow, I just manually added in the "multipolygon" tag and presto, it started rendering properly.  Took a few hours to complete, but I expect that.  It had been weeks before I posted the original question.

Thanks!