Boot Camp Issue with Mac Pro 3,1 and ATI Radeon RX580 graphics card
Most users have solved this problem by running Apple Software Update to get a newer version of Boot Camp. For users where this update is unavailable or unusable, there is a solution involving using the rEFInd Boot Manager. When properly installed and configured, rEFInd can allow Boot Camp to indirectly boot from Windows to a macOS residing in a APFS container.
The idea is simple enough: From Windows, you use Boot Camp to choose rEFInd. When the Mac boots rEFInd, the Mac will be instruct by rEFInd to silently boot Catalina.
For this to work, you will need use the following steps to install and configure rEFInd.

Create a small Mac OS Extended (Journaled) partition.
Download rEFInd. This is a free product.
Install rEFInd to the small Mac OS Extended (Journaled) partition. Basically, use the --ownhfs option when invoking refind-install.
Edit the refind.conf file to silently boot Catalina.

You should be able to install rEFInd without disabling SIP.