External_url changed after upgrade

I upgraded from 12.5 to 12.7 today, and the update seems to have changed my gitlab.rb file’s external_url value to the ‘public dns’ setting in AWS, rather than the assigned DNS entry for it.

Has this happened to anyone else, or know what would have caused that to happen?

This was indeed a bug in the 12.7 release.
It was tracked in issue https://gitlab.com/gitlab-org/omnibus-gitlab/issues/5018 and fixed in the 12.7.2 release.