In RealityCapture there is a restriction for ortho creation. Or, for the export of the ortho.
Limit for non-tiff options: both sides of a projection should be shorter than 40000 pixels. The JPEG images are not limited only resolution-wise, but the size of a file can also be limited. I was not able to find a specific limitation, but the highest value I found was 50MB which would not be sufficient for a 50000 x 36000 image. This is just figurative, but a 2-megapixel camera produces a 6 MB image. The 50000 x 36000 image has 1800 megapixels, which is 900 times more. That means that it would have at least 5.4 GB which is a lot for a JPEG.
To export large ortho projection a TIFF format, or in this case, a BigTIFF format should be used (possible when exporting TIFF).
As your project is not georeferenced, you don’t know the exact value for the pixel size. For your case it could be smaller than needed (as there are only general units). The estimated size is not always the best one (I suppose it is a lot of times lower as the GSD of the model, which is not necessary to go lower of that value).
I suppose it is not a bug, but system restrictions.
What do you mean by “separate”? Like resized region?
What is the camera focal length, distance from the captured object and the sensor’s size?