OK, and that's exactly what should indeed be the result without additional action being needed.
According to my tests if - after "extending video" - (picture bscap0001a.jpg) you (re)select under Video > Aspect ratio what is already selected there (so no real change), the picture will nevertheless change and become as you would expect it to be in the first place, i.e. like bscap0001b.jpg).
Like you mentioned this additional action should not be necessary.
And if I remember correctly, it wasn't in the past.
So like I said, I think you found a bug there which needs to be fixed.
(P.S.:
This Clone Wars movie isn't exactly 16:9 but that is irrelevant for this thread)