I can't swap the leftmost champ either.
same issue for me except it’s an issue with the left most character. Won’t let me delete and swap without pressing the info icon then backing out and trying to select them again. Also won’t let me click the boss to fight them.
Bugged after update.
Hit the information icon (i) for the left most champion, then tap on the champion and hit 'back'. That champion will be selected for replacement