Lightroom Classic: Export Tiff/PSD with Transparency to PNG with Upscale produces unexpected render

Yes. I start with this image from within Lightroom. It's an NEF file.

This same thing happens with images that already have the backgrounds removed as well. The image below had a transparent background, was imported into Lightroom with a transparent background and upon export, enlarging the image by 1000 pixels, it rendered this way:

I should also say, Lightroom Classic 10.0 being used on a Mac running Big Sur though it was also happening before I upgraded the OS on the Mac. It was not happening before I upgraded Lightroom Classic.

What version of Lightroom Classic did you use and what were your export settings?

Strange, I did a couple other photos and they both exhibit the problem so I got one good result out of 3!!

BTW, PNG files seem like they take a LOT longer to export than they should.   They take longer in Photoshop but in Lightroom they take an extraordinary amount of time to export.  

I found that as well, I think it takes longer because it's adding all that extraneous data to the file. I'm relieved that someone else was able to duplicate the problem - proof that I am, at least, not crazy.

@Rikk

I would add PNG to that as well as I've imported PNG files and tried to export them larger and it exhibited the same error. So - 

Exporting as a PNG larger than the original size exhibits the error. 

I don't know if it's important but I think there is a threshold of enlargement as well. When I was troubleshooting I could successfully export if the enlargement was small - when I enlarged the large side form 6,000 p to 7,000 p it did fine, but going up to 8,000 p the error was there. That has been my experience anyway. 

There should be a vid of the CSR attempting to correct the issue via my computer - Case number: ADB-16432718-F5M0 - if that's helpful and you run in those circles. I can't imagine it's all that exciting...