I accidentally discovered this while fixing Translate's REL1_31 tests. It seems that Translate's CI is running against the most recently tagged version of ULS, *not* the version from the correct branch, which is going to be deployed to Wikimedia sites -> therefore unbreak now.
Translate, via composer.json, depends on mediawiki/universal-language-selector: *, which pulls in the most recently tagged ULS version via packagist/composer. This gets installed in the normal extensions/ directory, overwriting the correct version of ULS that quibble cloned from Gerrit.
The straightfoward thing would be to remove the composer dependency, I don't know if that's acceptable to the Translate maintainers. In any case, not testing Translate against the right ULS version isn't acceptable IMO.