I can tell when this happens. The engine and transmission are fully warmed up. While the oil in the transmission is cold this effect does not occur. So I wrote to you a possible reason. You need to come to a specialty service on the transmission. There you may measure the pressure in the system. This is done with a pressure gauge (manometer). After that, it will be clear what to do next.
PS So you need to check the operation of the heat exchanger. If there are problems, the oil will be very hot and liquid.