4K footage on HD timeline is missing when scaling

Hi. First time poster. I've had a scout around but can't see an answer to my question. Please feel free to point me in the right direction if there is a solution.

I am editing a music performance - seven video feeds on a multicam timeline; mix of HD and 4K. I've done this a few times and I use the 4K footage as a crop option/zoom for different shots. It has always worked previously but it isn't now and I'm stumped. And desperate for a solution.

On my HD multicam timeline, after I have made all my cuts, I want to take one of the 4K cuts and scale it, but when I scale it the cropped image that was displayed just shrinks and the rest of scene just isn't there - replaced by a black box (see screenshots).

It's like all that data has just vanished. 

I have tried various scaling options (set to frame/scale to frame/scaling the source file) and nothing seems to remedy this.

Any assistance gratefully received.

Using latest Premiere Pro - updated today