So can anyone see what’s wrong with this image:
Answer: This prompt has no “back/cancel” button.
Arena (like Treasure Hunt) is one of only two game modes with no “misclick protection”, i.e. if you click the button by accident you cannot simply “back out” and return to the world map screen – you have to select all four troops THEN back out.
This is fixable, and easily! When the player is prompted to pick their first troop (the Epic troop), just make sure the Back button ( < ) at the top-left corner is still available for clicking (note how the prep screen under this prompt has standard “Back” and “Help” buttons visible already, you just can’t click them due to the prompt). This will help avoid misclicks, while not interfering (i.e. adding more clicks) whenever a player wants to perform an Arena run (for a Campaign Task or Guild Trophies, etc).