That small part of the frame is likely transparent - not white.
If you compare the spot removal in Lightroom with the same picture in Photoshop, they work a bit differently. In Photoshop it's possible to create a background layer so that the white area is part of the background. However, Photoshop doesn't have that feature. If there is white area it is essentially considered not to be part of the image and therefore cannot be integrated into the image using the spot removal brush. I have encountered that same issue myself in the past. And that's the only explanation I can come up with.
That seems like a reasonable explanation. But is it a reasonable design
The image frame in lr is always rectangular, but what is served by making
part of it unwriteable? By doing so the user is forced to export to
photoshop to do the job.
It's not a big issue, but it is deeply inelegant, sacrificing the options
of the user to provramming ease.
If you have created a panorama or an HDR image and there is whitespace, then the image is no longer rectangular. It is not possible to create pixels in empty space in Lightroom. For those images were filling in the whitespace is critical, that's what Photoshop is for in that instance. Maybe it isn't as elegant as you might expect, but that's the way it works with nondestructive editing.
I really question whether it is "not possible" . It could work otherwise
and to my eye, it would be in the end user's best interest.
That is, however, how it works now. Let it remain so for all that it