TYPO3 Core

Seems to work here: As stated somewhere, the topmost assigned group is default, and with the selector I see here, you can assign and sort groups any way you like.

works here too.
It was reverted to the old version. See ChangeLog:

• Ended the regime of the new be-usergroup selection box - back to the previous with ordering of backend groups since the first group was significant (being the default owner group)
  ...

Works again in 3.8