1. Add a (freestyle) job named 'foo'
2. Add another job named e.g. 'Foo'
3. If you click (and hold) "Freestyle project", you'll see "a job with this name already exists"
4. 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