Hi,

An overfilled transmission will affect the flow rate of fluid returning to the sump and there cause a quantity of volume to circulate.
The fluid level is an important consideration from high or too low.

It why it gets checked after it is up to operating temperature as the coefficients of expansion of materials, fluids or metals, make or break getting a prototype machine to work well enough to get into production.
Trust me I know a little bit about Research and Developing products as it takes a lot of homework up in engineering. From drafting and machining it becomes a hand in hand collaboration between departments.

Having a 3,000 rpm is not considered an idle speed.
An engine that’s Not under a load is approaching it maximum torque level and the transmission is having to displace that input energy through a bypass path that it shouldn’t for any long lengths of time.
With reduced flow comes a pressure, where as, moving internals absorbs that energy.
You may be actually overheating the fluid because you are not idling low enough even in Neutral or Park.
Yes I would expect still a fluid level problem arising.

Get the idling down.
Hoe were you able to drive or stop the car with that idle speed?
You failed to mention that tidbit.
Please Tell me, It must not do it all the time?

With todays cars you would have so many codes, buzzers or lights going off, the car might take a squat with you on the side of the road. (:-)

Phil