Bit of an update. The seatbelt was working again the next morning and it continued to do so for few days. Then it happened again (both times parked outside, hot sunny weather).
Luckily I had nothing to do that day, so I had it towed (Ford Roadside assistance) to my local dealership. The service manager ripped open the plastic cover, wiggled the seat belt and he fixed it. It is still getting replaced under warranty though.