The fabrication rules want that the foldings must be done perpendicular to the rolling direction of the metal sheet in order to avoid shearing.
Metal sheets are rolled during their manufacturing and generally you can see the line marks of the rollers.
After removing the spare wheel, take a mirror and check if you cant see inside some marks related to the rolling direction of the metal sheet. If they are parallel to the split it could be an explanation.
Check also the pressure of the hood on the sheet when locked.
