That reflects how things work on macOS and iOS right now, the scroll bar disappears when you stop scrolling. However, that does not apply to "pop-in" elements like the share pane. It operates like a slidable component on-top of the current app, it does not even have a scroll bar in theory, because from its point of view there is nothing to scroll. Whether that's a good choice to begin with is open to debate, but I assume Apple's design team would be averse to showing a positioning indicator due to the fact that it would break their skeuomorphism.