Can anyone explain what happened with this battle? How is it even possible to have this outcome?
Its a bug. One that’s been in the arena since its inception. When you switch out your dragons just before you enter the fight, the battle log will display them incorrectly (it’ll show the dragons that you had in the team before you made any changes and those that were switched out will be greyed-out)
(I’m guessing the following happened) Your opponent was probably doing a duty for astrid (the use 10 yellow dragon abilities). After completing went on to arena, and then perhaps rerolled until they found you. At which point he or she swapped out the yellow dragons for the dragons they wanted to use and then started the battle (which would then result in the battle log you made a screenshot of)
Ask for compensation, their game is their fault.