Rectangle Outline Appears on New Blank Sheet

[MDWR]

A new sheet in a Visio 365 file depicts a rectangle outline with odd top, bottom, and side margins. (This is not the first page in this file.) Cannot select the rectangle. Cannot remove the rectangle. I have selected "No Background". There should be nothing on the page. The page and print size are the same at 36" x 24". I don't know where this rectangle outline comes from or how to delete it.