I am also on macOS Mojave 10.14.6.
I get this issue sometimes when I’m using “Future” lighting. The explorer menu takes a while to drop down if my camera is actively looking at a spot with lots of lights, but loads fine once I move away. The entire issue goes away when I switch back to one of the other Lighting methods.
Other times, without “Future” lighting, this issue seemingly occurs at random when I’m in the “Play” test mode in Studio. I’m assuming Studio is choosing to prioritize the game graphics over the drop down menu?