The entire sensor is 50MP. By cropping the center quarter ("zoom"), you are capturing a 12MP image.
The image pipeline then upscales the 12MP resolution image back to 50MP, with no gain in resolution. Simplified, each pixel is simply copied to 4...
As long as the OP has the full cooperation of and communication with the original owner, it is the unsuccessful unbinding by the original owner that is the problem, and DJI should certainly be able to help with that.