Keynote slides not filling screen on external display

I have a persistent problem with Keynote and external displays.

When entering presentation mode, the slides never fill the maximum dimensions of the external display. A black band always surrounds the slide. This occurs on a variety of external displays and projectors. It appears to have most often when using a template or file that was originally designed in Powerpoint.

Changing the slide size on the document pane makes no difference.

This template must be used for these presentations, it cannot be redesigned from scratch in Keynote.

Keynote version 12.0 (7033.0.134) on MacOS Monterey 12.2.1 (21D62)