Mmmmm....I read back through the thread and you have fixed or replaced just about everything.
You mention it happens when just parked in the sun, so here's my stab in the dark...Do you have an aftermarket Master Cylinder with an adapter plate to the booster?
Some vendors use a plastic bushing in the aluminum adapter to guide the actuating rod. When things heat up in the front trunk, the hole in the bushing closes up and the bushing squeezes down on the rod, holding it in position. This is a documented issue that has numerous occurences and the fix is as simple as reaming out the bushing for greater clearance.
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