Thanks for the update on this. That is great information. Normally this is either a pressure issue (cut seal) or a broken component that is specific to reverse.
The way to find this is to first test the pressure in the unit with the info below. Then to drop the pan as you did and find what is in the pan.
https://www.2carpros.com/articles/how-to-service-an-automatic-transmission
Please let us know if you have other questions but thanks for the update as that is good information.
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Saturday, May 1st, 2021 AT 6:36 PM