New MacPro (2019) will be not able to see more addon card if the pcie pool is full used. You can try to manually configure adapter to pool A or B, then install our raid card after to see if it works. In our tests, the pcie management tool is not really smart, it may not aware of the status of the pool A sometimes.
The artical below explain how the expansion slot utility works.
Tags: ARC-1883x, Mac Pro, reboot