Comment # 75 on bug 978973 from Mircea Kitsune

Just a heads up in case this is relevant: Yesterday I was finally able to
switch from the old "radeon" driver to the newer "amdgpu" driver on my RadeonSI
card. This can be done by booting the system with the following parameters:

radeon.si_support=0 amdgpu.si_support=1

The splash screen bug no longer happens when using this driver. The gray
background and green ??? are gone, I always see the themed boot screen.

So this may either be a driver specific issue, either certain drivers take
certain steps differently which causes the screen to load differently as well.
For instance I also see the console more often during the booting process, so
it may very well be the later case.