I’m not sure about the gantry belts but the carriage belt is a strip, not closed loop. I think the gantry belts are the same.
The ends are clamped under an aluminum block that has grooves that match the belt teeth. The first time I removed the carriage I disconnected the belt.
I was glad to see the dust on the belt marked exactly where it had been clamped so I could reassemble the belt to the same length under the clamp.
It seems odd the belts would have stretched, they are very strong and fiber reenforced. I have a lot of time on this machine and there is not a hint of stretching. Maybe the belt clamp is loose and the belt slipped.
Another possibility is that the belt pulley slipped, they are adjustable. You can see they are mounted in an adjustable slot on the rail. The tool to tighten/loosen them is a 3mm Allen.
I would wait to hear from support for an official answer,
but it seems to me more likely a pulley or a clamp has slipped.