I also experience this problem, but after clean installing 18.8.1, the game crashes 'cleanly' or doesn't have a ghost window where the sound works fine but the actual game won't even show up. I just switched over about a week ago to AMD GPUs and these problems are very new to me, since I was previously using a GTX 1050, so I'm afraid I can't help you much other than trying to clean install the latest drivers from the website and possibly try clean installing fortnite. Also, by pressing 'ctrl + x' and pressing 'a', you can clean/ repair your systems by doing commands such as 'DISM /Online /Cleanup-Image /RestoreHealth' and 'SFC /SCANNOW'. Otherwise, just stick to 18.5.1 until the next fix for the drivers comes out.
Other errors I have noticed in fortnite are the glitched terrain / guns and invisible guns.
The terrain will go in an infinite line across the screen
Some guns will do this too (thermal when not scoped)
All guns when not from a chest, another player, loot drop, or loot llama will be invisible. If anyone has a solution to these issues, please say something.
CPU: AMD FX-6300 Six-Core Processor
Storage: 4TB HDD + 240GB SSD
Motherboard: Gigabyte Ultra-Durable (forget the exact model)
GPU: Power Color AMD Radeon rx580 8GB
PSU: EVGA 850BQ