AssetForge is probably generating thumbnails looking at the back of the models which wouldn't work with backface culling on. I spoke with Kenney about creating a mode that turned off backface culling specifically for this kit since it was annoying the use while placing the models and looking at them from the wrong side, but I don't remember it not generating thumbnails correctly. I don't remember testing this kit with AssetForge 2, however, and maybe the thumbnails are generating from the opposite side now which wouldn't render. As far as the other kits, I built them as blocks which was less efficient by far, but easier to place.
This kit is in standard wavefront obj and the scale is correct for Unity units which AssetForge 1 and 2 use. There is nothing wrong with the models. There is a forum on the itch page for AssetForge. I think there's a page specifically for this model kit on there. I recommend asking your question in that forum.