Since Arena runs take quite long, I think restarting the game ONCE per run is worthwhile.
Why?
To redo troop selection.
While afflicted by this bug and having to restart the game several times, it looked like troop selection always restarts from the first slot. Not only that, the troops on offer DOESN’T change.
Now that the bug’s been fixed, I started a fresh run, chose 3 troops and then restarted to have another go at choosing better troops to support the Epic troop that I really want.