I've made a video recording to demonstrate the bug and how it can be re-created.
I create a field group that are shown on posts only if an certain category is chosen. The field group contains a required field. The expected behavior is that an error message is shown if that certain category is chosen and the required field is empty, and that no error message is shown if that certain category is not chosen. That is, if the field group is not shown, there should be no test for the required field. This work as expected for all field types I've tested except the time picker-field. See the above mentioned video.