Why does swipe not work on iPad in sidecar mode?

I am trying to use my iPad (7th generation, OS13.4) as a 2nd display for my MacBook Air (OS10.15.7) via sidecar. It sort of works but is sometimes buggy (frozen screen on the iPad). The biggest issue is that I cannot scroll a document I open in "Preview" on the iPad except through the Air trackpad (very unintuitive). No finger gestures/swipes/... on the iPad seem to do anything at all. I CAN use the Apple Pencil to click on tools etc., but it is impossible to scroll with that in a controlled way (very jerky motion if I try to drag in the sidebar). It would help if I could display a full page of the document, and then just use the page-up/down buttons. But, alas, "sidecar" only allows landscape orientation (although, being buggy, it "accidentally" got frozen in portrait mode once.) Overall, I'm unimpressed - this could be a truly useful feature if Apple had put a bit more effort into making it user friendly.