This is likely a problem with the microswitch in the latch, not the regulator itself. When you let go of the handle with the door open, the motor is moving the window back up.
Symptoms of a bad regulator are generally bad action or inaction when action is expected, not the reverse.
The issue could be displacement of the latch microswitch, problem with wiring to the switch, or the switch actually being bad.
You can eliminate everything except the above by jumpering the switch. I don't have the wiring diagram handy for your MY but on the earlier Boxsters, this would be a matter of grounding the relevant pin on the connector in the rear trunk with the door open and verifying that the window drops.
As a quick test, you can fuss with the door latch with the door open and see if the window goes down. If it does, then the problem is mechanical, not electrical.
Edited 2 time(s). Last edit at 07/08/2021 10:59AM by Boxsterra.
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