Hit this on my Studio last week — for me it turned out to be the "Group windows by application" toggle interacting badly with the new space preview render. Toggle it off in System Settings > Desktop & Dock, then back on, and the thumbnails come back on the current space.
If that doesn't do it, killall Dock (or a logout) rebuilds Mission Control's window tracking and the previews reappear. In Beta 3 R2 I've had to do that once after waking from a long clamshell sleep — thumbnails go blank until Dock restarts.
One other thing worth checking: System Settings > Accessibility > Display > Reduce Transparency. When that's on in the current beta the thumbnails render, but at near-zero opacity, which reads exactly like "no windows visible" until you tilt the display.