Arena, Triumphant surge without legal target
What you describe was certainly a bug in Arena. You can't cast a spell if you can't choose legal targets for it. By the rules, the spell will not hang around until there is a legal target.
Spells and abilities that require one or more targets can't be cast, activated, or enter the stack if you don't have a legal target at the time of casting/activating/triggering it. A spell that was started to be cast would be returned to where it came from, usually the hand.
The process for casting a spell, activating an ability, and triggering an ability are mostly identical for the purpose of target selection, so I'll only quote those rules for casting spells.

601.2c The player announces their choice of an appropriate object or player for each target the spell requires. [..]
601.2e The game checks to see if the proposed spell can legally be cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 722, “Handling Illegal Actions”).