The preview window stays completely black for markdown files until – I assume – all referenced images are loaded and ready to display. With larger files this can result in longer wait times. This behaviour is a regression from DT2 that seemed to render the files progressively as every other browser out there since ages.
Desired behaviour: revert to progressive rendering (as in DT2)
This has already been reported this morning here:
https://discourse.devontechnologies.com/t/dt3-b2-document-display-flicker/47572/7
No, that’s not the same problem. I am referring to the long wait time before any content gets displayed.
You stated this…
With larger files this can result in longer wait times.
The issue reported certainly appears to be the same but only being shown with a smaller document in my example.
In any case it is not solved by resetting the prefs. 
Do you use Dark mode? Then it’s actually a workaround to reduce the flickering (DT3(b2) : Document Display flicker) caused by the WebKit, without the workaround the flickering is even worth
No, no dark mode. And the flicker is not a problem. But the waiting time until I start to see anything is.
Beta 3 will revise this (as long as you don’t use Dark mode)