Hi @RMBuck - While I do not have an iPad Pro, I double-checked this with my iPad (6th Gen) running 15.0.2. When I open a submission (I tried a variety!), I was able to change from portrait to landscape and back easily.
The sidebar closes automatically when I'm in vertical mode. When I tap the light gray line above comments, it expands first to the halfway point on my screen. When I tap it again, it fully expands. Tap a third time, and it closes.
However, I see where it could be helpful to be able to hide the side menu when I am annotating AND my iPad is in landscape view. This hasn't ever been a feature, but I bet a number of instructors would enjoy having it available. I think it would be worthwhile to share a new idea conversation! If you author an idea, please link it back to this conversation. That way, when other Community members search for a solution, they'll be able to piece everything together.