Note, that the mentioned tscon command does not close your UI session, but switches it to admin console one which can have different screen resolution than yours. I am not sure about the details, but can imagine the situation, when the size of the browser window remains the same and thus, on the lower screen resolution, part of the window appears out of desktop viewport, thus is not captured and creates impression of partial screenshot.
Does this look like the reason?
Another note: As you are working with web browser and if you are not interested in browser's window itself, you may consider .PagePicture() method instead. This method takes screenshot of complete web page scrolling it automatically.
I tried with the PagePicture method but the result is the same it shows partial image. As, I am using tscon it command it shows desktop console of that virtual machine, so not sure if it has become low resolution.
I have attached the file to show how the images looks with connected mode and disconnected mode.