I have had this error too (Im the other user Elaine mentioned lol), If you click on the material image for the glass you can change the parent that the material instance uses, Use the "Master_Masked" for objects that need tranparency (like fences) and "Master_Translucent" for objects that need tranlucancy (Like glass and headlights). If you click the wrong one by accident and it breaks fully you can get it back by setting the right texures back in the Parameter groups above the options in steps 2/3. I plan to write a full guide on it soon (I work full time so, it will be done when I can get time sadly)

Hi David Kerman another user has reported this and it's on our radar!

Please see this post https://kitbash3d.canny.io/cargo-feedback/p/broken-opacity-mask for a fix.