Second 4096x2160 4K monitor is only showing approx 66% of 4096x2160 image

I have two full 4K monitors (4096x2160) and am running Windows 10 Pro.

It's a year since I worked with Adobe Premiere Pro CC and things used to work fine if I just went into Settings-> Playback and selected one of the monitors to be the playback monitor.

A year on, I am now on the latest update and trying to familiarise myself with what worked fine last year and it seems something's broken the second monitor playback.

I've imported several 4K clips from a Canon EOS 5D Mark IV and a DJI Osmo on the main application running on my main 4K monitor within the Source Monitor panel with "Fit" selected everything looks as it should. The Project folder shows all clips under "Video Info" as 4096x2160.

When I go to set the second 4K monitor I am offered choices of: Adobe DV (with a "Setup" clickable link), Adobe Monitor 1 (4096x2160) and Adobe Monitor 2 (4096x2160). All good because this matches the clips.

But when I select either of these options the monitor only displays about 66% of the full image (with the left most and bottom 33% being chopped off. It's as if the software thinks it's got monitor of 3000x1500 resolution or something and can't fit the whole thing on screen.

It's making the second monitor completely unusable. Can anybody suggest how to fix this so I get the full 4K resolution on the second monitor instead of just part of my image as if some sort of zoom setting were being applied?