problem with Radeon Pro 580 in Boot Camp Windows10

I have a 2017 iMac 27" with the Radeon Pro 580, 8G of VRAM.

I have the latest drivers through the Mac update. 21.19.512.1

I've also tried the latest Crimson drivers modified from bootcampdrivers.com

No difference with my problem.

 

No issues when in MacOS.

 

I run Windows10 via Boot Camp.  When I'm in Windows, the System Information tool shows "Adapter RAM (1,048,576) bytes". That doesn't seem right for a 8G card.

 

Any ideas what could be causing this or how to fix it?