I had the same problem too. I managed to complete the mission by:
Reaching waypoint 3 (the release point)
Flying back to waypoint 2, which triggers 31 finally to leave waypoint 1
Then you can rejoin and continue the mission.
Or maybe the trick is to just wait for ages. Here's my trackfile (contains spoilers, and excuse the multiple RTBs to rearm, I needed extra JSOWs).
Track file shows it took 42 minutes into the mission for 31 to leave waypoint 1!
Tacview-20230102-015405-DCS-R1 M11.zip.acmi