I don’t know if anyone else has encountered this issue, but it just cost me a few hours and I couldn’t figure it out for ages, and the solution that ultimately worked for me seems weird, so I figured I’d put this here just in case someone else comes across this problem!
For me, LineSegs (no matter what thickness and colour settings) would only render as a few unconnected pixels, creating something looking like a starfield. I figured it has something to do with the OpenGL support on the raspberry PI the app is running on (it was working fine on MacOS), but no amount of changing rendering backends etc helped.
What helped in the end were these settings in Config.prc:
framebuffer-multisample 1
multisamples 4