Continuity Markup now showing in Preview

I have an iPhone XS max with iOS 13.5.1 and a MacBook Pro mid 2015 with macOS Catalina 10.15.5. That should satisfy Continuity Markup system requirements. I can do Continuity Markup with Quick View, with Screenshot's Floating Thumbnail, but, surprisingly, not with Preview. The icon is simply not there to click. Any suggestions how to fix this? Both devices were recently restarted.