Recently I was attempting to create a UGC concept, and it came out alright, however upon exporting it into the game, I faced an issue that I can only imagine would be with normals.
Check the Double Sided property box in your meshes Properties, it makes your mesh have 2 faces, basically double sided. It does take up computing power, but not that much.
Try selecting the faces with flipped normals in edit mode and then search “flip normals” and hit enter, it should fix it if the faces normals are flipped.
I believe all you need to do is fill in the hole at the bottom of the cap. Roblox doesn’t register backfaces, so you will need to fill it in.
explanation of your problem:
blender: filling in a hole
Select the borders of the hole (sometimes you can select them as a loop with Alt and right clicking). 2. Press Alt + F to fill the hole with new geometry.