In XFRM 2.3 RC 3, when I add or edit a resource category in the admin CP, I select a forum in "Automatically create thread in forum". Even though this forum has thread prefixes, none are shown in the "Automatically created thread prefix" drop-down menu.
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XFRM release (2.3.0 Release Candidate 5).