Hmm. Genuinely not sure. A few things to check:
Normals
Go to blender, viewport overlays and then face orientation. Blue good/red bad.
If red: tab to edit mode, a, alt n, flip.
UV
Again in blender. Edit mode. u, "Smart UV Project"
Transform Matrix
Object mode. Apply, All transforms.
Reimport to Godot.
Transparency
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.