I don't know if Apple has come up for Bootcamp drivers for AMD or not. but this is from an earlier thread about Bootcamp drivers and AMD gpu's.
It is an old thread, so it may not be reliable.
NOTE: copied this from Radeon Pro Driver download release notes:
Packaged Driver Version: 16.50.2001
This driver is not intended for use on AMD FireProTMand Radeon Pro products running in Apple Boot Camp platforms. Users of these platforms should contact their system manufacturer for driver support
I’m referring to drivers that have been correctly modified with Mac graphics cards settings to work with the new versions.