When I export my blender work as an OBJ file, and then upload it into a mesh on Roblox studio it isn’t the same. (Also I saw a lot of people have the same issue before but I can’t remember what I have to do myself so this can get annoying as it’s a repetitive topic.
This is a common problem users encounter when importing .obj files into studio when the direction of the model’s normals isn’t paid attention to—some of the faces are inverted, and you have to press Ctrl+Shift+N with all faces selected in edit mode and make sure that the new dialogue that pops up in the bottom left of the viewport has ‘Inside’ unchecked:
You can visualize the direction the normals are facing by selecting this in the top right of your viewport; blue means it’s facing outward (correct), and red means it’s facing inward (bad in your case)