I have the exact same APU the ryzen 5 2400g and the exact same problem; freesync only works on windowed mode. Please let me know if you found a solution.
I've really only tested it on rainbow six siege. Also my monitor is a Samsung s24f350 60hz and it supports freesync obviously. I have windows 10 pro. My mobo is the asrock ab350 pro4. 8 gb gskill ram @ 2666hz