Full Control is missing a critical stage
In live games, there is a critical step in game states where instants may be played that is not reflected in Arena: A creature or planeswalker has fully resolved, but has not yet activated any abilities. This state is the only one in which there is counterplay for certain Planeswalker abilities in certain deck types. Since we can't put the Planeswalker genie back in the bottle, we need to be able to act during this phase, as well as being allowed to act at the end of a phase, before the phase fully changes.
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Rezzahan#77802 commented
When a creature or planeswalker spell is cast in the normal way (with sorcery timing), and the spell resolves, the caster (who is the active player) receives priority first. And if there is nothing triggering of of the event of that creature/planeswalker entering, then they can use stuff with sorcery timing before you get priority to do anything about it. This is not a bug, this is how the rules work.
Also, no step of phase ends without all players passing priority in succession when the stack is empty. But that also means, that the active player cannot wait for the opponent to act, and if he does nothing, do something. When you are the active player and the stack is empty, you do your thing or risk not being able to during that step/phase. Because if you pass and the opponent does nothing (thus also passing), and there is nothing on the stack, the game moves on to the next step/phase. Just as the rules say it should.