Not sure if this is by design or a bug, but if you create a field as a YouTube type and click the radio button for Display options >> Display to "Under the item content", then save, it will return to "Above the item content" being selected and will continue to display as such.

For me is the same

Thank you for bringing this issue to our attention! I can confirm this should be further reviewed and I have logged an internal bug report for our development team to investigate and address as necessary, in a future maintenance release.