The Surface family includes Surface Pro, Surface Laptop, and other Surface products. Surface is designed to help organizations, students, and consumers be more productive.
Hi,
We would like to know if there were multiple apps left running in the background before the Surface Pro 3 went to sleep. If so, can you try to close all apps before leaving it to go on sleep mode and check if the issue persists? This will help us determine what's really causing it. In the meantime, we suggest that you uninstall and re-install the keyboard driver in Device Manager and see if that helps.
Let us know the results.