I believe the documentation is saying that it’s only capitalized when displayed in the UI, and the raw value should be in small letters. Since the stated purpose of this feature is: “so you don’t have to deal with uppercase strings in your .travis.yml file”.
So they actually fixed a bug rather than introduced it.
An inconsistent display reported by @rjollos, however, is a genuine UI bug.
@rjollos, I see that your fork has Build Config Validation enabled while the base project doesn’t. This may be what’s making the difference. Could you disable it and see if that changes things?