I have the same iOS and app version and obviously was able to do it.
hmmmmm
have you tried force quitting the app and relaunching it? maybe restart the phone ?
While still not ideal, I made an update to this process to make things a bit easier for desktop users in the last week.
https://www.on3.com/boards/threads/browser-based-add-on-to-sort-a-thread-by-staff-posts-only.8566302/