The object I see when I open your file looks fine. It reports solid in both Entity Info and Solid Inspector². The problem will be the other stuff in your file. You have some hidden components. If you exported with Free, I don’t think you have the option of whether to export hidden geometry so that might be the problem.
Without an outliner window, you’re working blind so you’d have to remember that the stuff was still there, or turn on hidden geometry to see it.
I deleted all the hidden objects and purged unused stuff. This reduced your file size by 99.8% ! Your file attachment wouldn’t work as it was too big for the forum, but if you’d trimmed it down first, it would have worked fine. Not everyone wants to download such big files either, especially when most of it wasn’t actually the content that mattered.
ModularMap_gm.skp (223.3 KB)
Your other file had one stray line in the object that was visible. There was a similar huge reduction in file size after deleting the hidden objects and purging.