Normal for 10.14.5 and Radeon cards...
I wonder if this issue affects real Macs too.The new AMD 10.14.5 drivers do this for everyone and so far, no solutions.
As those cards only fits in MacPro's, being PC GPU's, bootscreen is not visible on real Macs. Less chances that it'll be fixed. Mac cards needs special vBIOS to display boot process...I wonder if this issue affects real Macs too.
As long it is just a cosmetic issue, OK. But if that also have effects on the overall macOS , less good