- Add a (freestyle) job named 'foo'
- Add another job named e.g. 'Foo'
- If you click (and hold) "Freestyle project", you'll see "a job with this name already exists"
- When you release the mouse button it turns into "This field cannot be empty, please enter a valid name" (see screenshot)
My guess is, this might be connected to the handling of case differences in Jenkins job names in general, e.g. https://issues.jenkins-ci.org/browse/JENKINS-37761