TL;DR: Down Ramp foundations has been locked out of any other build mode but curved. If there's another way to contact the Mod Dev let me know.
Love the mod, but want to let the author know about this, hopefully it can help. My Down Ramp foundations can no longer select any other build mode except curved. All 3 versions of the Down Ramp are affected, no other foundations are. I've been building a road with the mod to make curves, but (ironically?) haven't used the curved mode for the Down Ramps. When I hold the mode select button the Radial menu only shows Curved.
I tried disabling all the other Mods (Infinite Zoop and Vehicle Path Viewer) and that didn't help.
I tried disabling Curve Builder itself, which resulted in no build modes being selectable at all for Down Ramp, not even the vanilla options. When I held the key, I didn't even get a radial menu at all. The last picture is me holding down the button, even though nothing shows up :p