Build 411. The previous build did not have this issue.
In a blank 8-bit (legacy) Vegas project, I've added a color gradient via generated media, and to that I've added a levels fx, which I've used to darken the blacks a bit:
Now, I've added a second levels fx, this time choosing the computer-to-studio template. As the scopes indicate, the levels have been correctly converted to 16-235:
Now I've changed project properties to 32-bit (video levels) mode:
The waveform scope is correct (I verified this using photoshop). Instead of mapping all values of zero or lower to 16 (as in 8-bit mode), VP 411 is doing some kind of weird mapping of super-blacks such that we end up with values below 16.
I *think* that values at zero are still being correctly mapped to 16 (and similarly with whites), so that there isn't a density shift overall...but I wouldn't bet on it.