Why Will 2012 Mac Pro will work with 2 graphics cards, but not one?
It appears the problem was that FileVault was turned on. Even without Mojave, some graphics cards without special firmware (although approved by Apple), are unable to render when in FileVault. This is why Apple says to turn it off before moving to Mojave, but they do not mention that it will not work with their new graphics card recommendation. Therefore the order is very important


Turn off FileVault
Install new card
Update to Mojave