When a Heisted card is in exile, the Arena client doesn't give priority if you don't have the available mana to hard cast a card, even if it is possible to cast it via it's alternate cost.

Ex. I heisted a Treasure Cruise in my second main phase and could pay to cast it using Delve, but the client skipped directly to my end step

Ex. I heisted an Archive Trap. My opponent searched their library with a Polluted Delta on their end step and the client immediately went to their turn, instead of passing priority so I could cast it with its alternate cost