Nope, Rocket is not why my fonts are not respecting the weight setting, because I am repeatedly emptying the cache or even turning it off trying to make the latest Cornerstone work. Furthermore, as mentioned, the font weights DO respect the button under Typography, and the changes appear live in the preview. It’s just the button in the Navbar section that doesn’t seem to work, either on the front end or in preview, with Rocket on or off. It seems very much like a bug.
If you think that Siteground caching is affecting selected buttons in the Options, well I’ll try turning that off and see what happens, but it seems odd that it’s selective!