Illicit Masquerade does not let you choose resolution order
Even with auto-resolving disabled, the order in which multiple creatures go to the graveyard to trigger multiple Illicit Masquerade returns is not controllable.
Example 1: I have both Snarling Gorehound and Unscrupulous Agent on the battlefield with imposter counters. Those two die at the same time. If it resolves the Agent's trigger first, that exiles, and I can choose to return the Gorehound. (Yes this is the legal play, and it functions that way.) The problem is that if I instead wanted to resolve the Gorehound trigger first and return the Agent to play. Arena does not let me make that choice.
Example 2 is worse: an Agent in the graveyard already when a Festerleech and a second Agent both die at the same time. If the Festerleech resolves first there are 2 Agents in the graveyard to choose to return, but no way to distinguish them! If I pick the one that was put in the graveyard to trigger Masquerade, the second Masquerade resolution can return the older one. (yes the rules do that, and yes arena gets those rules correct.) However, If I end up picking the one that was already in the graveyard, the second Masquerade resolution has nothing to return - a difference between 1 or 2 Agents on the battlefield when I'm done!
All this makes the card significantly less powerful than in paper magic, and probably makes the card entirely unplayable with the possible randomness.