Hmm. Genuinely not sure. A few things to check:
Go to blender, viewport overlays and then face orientation. Blue good/red bad.
If red: tab to edit mode, a, alt n, flip.
Again in blender. Edit mode. u, "Smart UV Project"
Object mode. Apply, All transforms.
Reimport to Godot.
In godot, select MeshInstance, in inspector select Mesh, Material, Flags - ensure transparency is off.
In fact, remove the material (save the old one if you need it) and make a new spatial material.
Z-Fighting / Frustrum
Select camera. In inspector reset Fov, Near and Far if changed. Ensure projection is perspective.
If all of those don't fix it, I'm stumped I'm afraid.