Large dng pano not rendering at 1:1 zoom in Lr 9.1 Classic Library module

I have a very large pano made in Lr from 19 vertically shot raw images from a Nikon D90 camera. In the Library module of Lr 9.1 Classic with Loupe View, this is what I see:



Clicking to zoom in at 1:1 view I get a very low res image while Lr is loading:



Once loaded, the image area goes grey - 1:1 image view is not rendered:



Zooming back out to Fit View briefly flashes the low res image, then the image is displayed as normal, like the first screen shot above. Clicking to zoom in again at 1:1, first gives a grey outlined box:



before again showing a grey image area like the third screen shot shown above.

In the Develop module, zooming to 1:1 view works as expected.