I haven’t had this problem for quite a while now on my Surface Pro 3, Windows 10 pro 64 bit. Host is also Windows 10 Pro 64 - both fully patched.
I wonder if uninstalling and re-installing GoToMyPC on both host and client might help? I did set the preferences to use host cursor as per the original suggestion. Routinely I use "scale to fit", as my host PC is 1920x1080 and the Surface doesn't match. However, sometimes I also use it with a second 1920x1080 monitor on "full scale" and that works too - but in that case I set the Surface font to 100%, not the default 150%.
However, I didn’t consciously do anything – it just seems to have resolved itself.