Originally Posted by HakuYowane

Well actually, when you have a raid boss available, your chances of getting a bonus stage are the same, when the game decides to give you a raid boss but its not available it will give you nothing.

Also if you have an expired bonus stage for whatever reason this will cause that you won't get any more bonus stages of the same difficulty until you refresh the page.

at least it lowered the chance of you getting Raid Boss from normal mission.