OK.. update. So after fiddling with it some more, I think I've pinpointed the source of the problem. It's related to the wire harness on the right side of the boot lid.

By simply manipulating the rubber boot and the wires inside it, I was able to recover normal operation.

However, visually I see no damages to the wires, and even worse, the problem can come back. When the boot lid is completely open, if I keep manipulating the wires/rubber boot, eventually the lid can close without my pushing the button.

Anyone have a similar issue?