I'm guessing so those aren't road blocks to getting 5 year mission. However, I would presume (and I've not tested this) that if you can complete it in one pass, you'd get a gold or have a chance at gold depending on the combos, whereas you'd get lesser or no medals for multiple attempts. This keeps an incentive for doing better while not forcing you to, say, play PD3 six times in a row until you finish it.

Another reason might be to limit the points you can get.

This still has the auto-launch bug, where if you're holding down the left flipper and then press the right one, the auto-launch fires. This usually happens at the start of a mission but I haven't pinpointed the exact details of how to active it, but I know others have mentioned it. I'll update this if I can find the sequence.

Just downloaded 1.61.
When the ball is in the scoop, if you press and hold the left button (maybe right as well but don't know) and then press Punch It the auto-launch will fire. I believe this can only occur once per ball. It's very similar to the both flipper auto-launch, although I disabled that option in the settings.