Lightroom Classic CC 7.2 Develop module, two monitor display Low Res image

Hi

Lightroom Classic CC 7.2 Develop module, two monitor display.

In develop module on my left monitor (i.e. main window) it's showing what appears to be a low res version of the image (see stepping on nose on left). Canon 5D mk iv RAW file.

The right hand monitor (i.e. secondary display) is displaying the full res version.

If I switch to library view it switches round (low res on right, proper res on left).

Any idea what's going on here and how I can fix it?