Glitch or bad design during booting on Mojave?

The firmware controls that phase of screen rendering, not the OS.

My understanding is that by design, the full graphics library mode isn’t enabled intentionally. Whether people label it a glitch or feature (or good / bad design) might be wading into opinion territory. I don’t know the back story on this, but it’s plausible an engineer figured the risk (or manager figured the priority/cost) to embedding more code into firmware when weighed against a “let’s be pretty about everything” design goal. We can all agree on what shipped though by inspecting the screen during updates.

We are not seeing any problem with the update at work so haven’t reported this as a problem to Apple.