I've had the same issue! This is how I fixed it.... Uninstall Radeon Adrenalin software and also use the AMD clean uninstall utility AFTER uninstalling Radeon Adrenalin software.... The clean uninstall will remove old drivers and junk from your computer AND it will even set you up to install the latest version of Radeon Adrenalin software 18.9.1 (Release date 9/12/2018 Latest) which can be found here ----> AMD Ryzen™ 5 2400G Drivers & Support | AMD
I just did this just a few hours ago and as far as I can tell all of my games are running smoothly now 60FPS or more!! Fortnite I can even max out the settings now thanks to overclocking with the Ryzen Master software. I'd prefer overclocking with my motherboard's BIOS although, most motherboards will be a hit or miss (especially B350's) weather it will actually work properly or not. So I HIGHLY recommend overclocking with Ryzen Master even though it's more tedious to do and some settings you have to re-apply after a reboot or a hardware change but, it will definitely save you in the long run while your'e waiting for us to improve our drivers and software... We are trying our best too troubleshoot as many different hardware configurations as possible and we'd love to hear your feedback!
Wish you the best of luck
Feel free to contact my email on my profile page for more support if needed...
- Parker Thompson [Official Ryzen™ Troubleshooting Executive]