My opponent had one creature on the battlefield and cast Cost of Brilliance. I played Return the Favor to change the target of the player to me for drawing cards & losing life. When it came time to select UP TO ONE target creature to put a +1/+1 counter on, MTG arena would not allow me to not choose the opponent’s creature or any creature. I was in danger of timing out my action, so I chose “cancel” hoping that it would cancel that part of the spell, but not cancel what I had already selected as myself to draw the cards & lose life. Instead, what happened was that all the effects of Cost of Brilliance were cancelled, and that I cast it for nothing. My opponent was able to draw the cards & lose life, as well as put a +1/+1 counter on their creature, and I was out one less card in my hand because I cast it and it did nothing.

This is playing on the latest client, on iOS version 26.5. I did not get a screenshot showing I couldn’t choose to not target a creature, but I got screenshots showing the spells cast in my graveyard and my opponents graveyard.