Search for BMW goodwill. The dealer has the ability, in some cases, to cover issues just outside warranty.

The transmission contains a pump. This pump pressurizes ATF that flows through the valve body and causes actuators to move.

When you increase the engine speed, you increase the pump output...this can then overcome the leaking of fluid. Hence at low idle it is showing a problem, at high idle there is enough pressure.

A transmission shop (NOT BMW dealer; NOT BMW Indy) will connect pressure gauges to an AT and monitor shifting pressures, to help diagnoses.

You keep taking it to BMW shops and car places, I expect they will never find this issue (if it is trans)