priority
I was attacking with Quintorious, Loremaster. My opponent blocked with two creatures. I activated Quintorious's ability, targeting a removed Elspeth's Smite. Quintorious's ability resolved, targeting one of the blocking creatures. They cast a spell that would allow their creature to not be effected by Elspeth's Smite. I had Surge of Salvation in hand, and, if cast, it would have allowed my Quintorious to survive the combat. After their spell resolved, I should have had priority to cast Surge of Salvation (I had the mana available) but Arena did not give me priority. Arena has always given you priority in combat to activate combat tricks after the stack resolves, so I do not know why it did not do so on this occasion.
The resulting boardstate was very different than had the bug not occurred.