A safety issue has been identified in the SEAT Arona and Ibiza models that can cause the left rear passenger seatbelt to come undone when carrying five people. The issue also affects the VW Polo as the cars share the same MQB A0 platform.
The issue was discovered by Finnish car magazine Tekniikan Maailma. They have said the rear-left seatbelt socket rests below the middle belt’s socket. In their testing during a high-speed lane change, the middle socket pushed down on the rear-left seatbelt’s release button, which in turn unlatched the belt.
You can see in the video below.
SEAT has confirmed the issue and replicated it at the factory. They have advised drivers to not use that seat in the car until a formal recall has been put in place. Expect existing owners to be notified by SEAT directly.