So I was trying to pass my mesh from Blender to Roblox Studio, but I got some problems, so when I transform the MeshPart to the actual Mesh It gets an effect which makes some part of the mesh invisible. Here’s the video:
Here’s how it is on Blender:
I don’t know if you can help me up here, but I appreciate it if you write down here something that may fix it, even if you don’t know, and if you know I’ll appreciate your help and I’ll be very grateful to you! 
You have your Normals flipped on some Parts.
In Blender go to the Edit mode, click the Mesh tool just to the right of the Edit mode tool, and in the drop-down menu you should see Normals with an arrow beside it. Put your mouse on the Normals tool and a menu should pop up with Recalculate Normals. Click that.
You can also go to the right side of your screen and click on the Show Overlays tool (small icon at the top of the screen with 2 circles (1 is a circle outline, the other is a solid circle) with an arrow beside it. Click the arrow and you’ll see a Viewport Overlays menu. Click the box next to Face Orientation and this should show you orange faces for the ‘inside’ of each face and blue faces for the ‘outside’ of each face.
You can select the orange faces that should be on the outside and go to your Mesh>Normals>Flip tool to flip the Normals for those faces.
You also seem to have a lot more vertices/triangles than you need. The less you have the better it will be for rendering in your game.
edit: my example is for Blender 2.9
