This has happened twice. Each time i beat belial, went into Act 3 games several times. But upon changing my quest to Act 1 shattering crown, i was unable to choose Act 3 first quest , as it was unavailable. I am left with only the begin Lord of Lies quest.
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have had other 3 friends test the above i have stated, and they all were unable to access Act 3 inferno after changing their quest to a previous quest such as Act 1, shattering crown.
BTw i did get the achievement and the reward for a flawless belial kill, and i did post this in the bug forums, however i wanted another way of making sure your team is aware of this issue as the forums are flooded with spamming of general issues.