This problem was noticed when Anti-Harassment deployed Special:Investigate to svwiki. Swedish translations were included in 1.36.0-wmf.10, but the English translations were appearing on the page even after the svwiki was running wmf-10.
One of the new messages was 'checkuser-investigate-tour-targets-desc'. Running echo wfMessage('checkuser-investigate-tour-targets-desc')->text(); on the server gave the Swedish translation, but ResourceLoader was serving the English translation.
@Krinkle fixed the problem by running:
MessageBlobStore::clearGlobalCacheEntry( MediaWiki\MediaWikiServices::getInstance()->getMainWANObjectCache() );
Apparently the localisation caches of downstream services are being purged just before the new translations are synced to the servers, effectively delaying new translations until just before the next rollout.