The level preview isn't taking into account changes made in autoInit or any script associated with a group run with an atStart trigger.
Have a look at my recent changes to The Sunken Ship for an example. The water should be translucent, but it's opaque.
I'm pretty sure this used to work correctly. It's a little trickier to see, but have a look at the preview for Quick Dive. The fuel sappers are not highly visible. However, if you save a copy of the level somewhere else, the fuel sappers will be prominent in the newly generated preview. So I think it was working correctly whenever the last edit was done to that level and that preview generated.