I don’t think that’s an issue, are you seeing a warning or error?
The thing is that for several knob types, Rack uses multiple images (to give them a “depth” kind of look), but a “flat” style shows up better on the MM screen, so we just use a single image. If you look elsewhere in componentlibrary.hpp, you’ll see most/all knobs derived from app::SvgKnob also load a file ending in _bg.svg, which also is not present in the assets/rack-lib dir.
Just a warning in the console, saying it was missing. I thought this was the cause of seg fault but it wasn’t related, so I don’t think this issue is a big deal.