The upgrade (of some plugin) had been done via docker_manager, so I destroyed and restarted the container to revert to the older version.
So maybe what happened is that upgrade got ember-cli turned on when it hadn’t been in the last rebuild?
I checked and see that EMBER_CLI_PROD_ASSETS: 0 is set in web_only.yml. MIght docker manager have done a rebuild with ember-cli turned on?