logstash (https://logstash.wikimedia.org) reveals that on the WMF cluster, there are some "GettingStarted requires CentralAuth to be installed" on wikis which definitely have CentralAuth installed.
It doesn't *seem* common (there are only 146), but I could be mistaken (e.g. it rotates faster than I thought), or it could be a new issue.
A quick test revealed that it redirected back as expected, so it's not a universal issue.
Version: master
Severity: normal