In the original game from which I pulled Milliways for glowfic, it was up to the individual author but generally people would find that the door was mysteriously stuck. So mysteriously. So stuck.
I would say “whatever works in the situation”; the Milliways door isn’t bound to follow standard rules. Mysterious stickiness might be a reasonable default, though.
Is the difference between Milliways’s not being bound by usual rules and Materia’s anticausality that Milliways generally does stick to some rules unless there’s a reason not to, that it’s understood there’s a being in control of the exceptions to the rules, or what?
When Milliways breaks its rules, it's because it had a reason. When Materia breaks its rules it's often because it hates you, and they won't go back to being rules afterwards on a consistent basis.
When you get a door, you walk out back to where you were, not the other side. Would the door potentially reappear right away when you open it, so you still can’t get to your destination?
I’m thinking of somebody who gets Milliways doors iff they would otherwise be going somewhere they’re unauthorized (ranging from as innocuous as trying to use the wrong bathroom to as severe as not being able to enter an enemy compound because Suddenly Milliways).