Aha! The guy who handles updating my site (me) changed me to tests-passed and now all’s well. It looks like tests-passed is not the default in spite of claims to the contrary.
Well, a comment in containers/app.yml claims
## Which Git revision should this container use? (default: tests-passed)
But uncommenting
version: tests-passed
changes which version I get, so apparently that’s not quite true. If anyone else is interested, more info is in the Change tracking branch for your Discourse instance.