Glitch or bad design during booting on Mojave?
The firmware controls that rendering process and it’s by design not using the full graphics library so it’s hard to say if it’s a glitch or hardware issue or if an engineer figured the risk (or manager figured the priority/cost) to embedding more code outweighed the “let’s be pretty about everything” design goal.

Time will tell if Apple patches that or if others can reproduce it on their equivalent hardware and then you’ll know if it’s hardware defect, by design and if that design got someone to tweak the code / drivers to remediate that visual artifact.