When in presentation mode, papyrus (or android, perhaps) letterboxes or pillarboxes the tablet to fit the presentation display.

My personal perspective is that the whole point of the presentation is the viewers, their view should take priority, and the tablet view should be boxed. If there's no user setting, then that should be the default.

My current workaround is to present only PDFs and instead of creating a new page, I import a blank 16x9 pdf which properly appears on the casted display.