Tubeless tire became leaky
It could be anything. Submerging the wheel in water will be useful. You might need a new valve, new rim tape or a new tyre. The air may be leaking around the spokes, around the valve, through a hole that is not properly sealed or just through the tyre material (least likely).
Be aware that a hole that is not properly sealed might be leaking air only during the ride when the tyre material is being deformed.
I do not thing you can get a definite answer without testing the real physical wheel.