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This happened a few days ago with my 2022 NX. There are no obstructions between the HUD and windshield. The black spot moves when the HUD is repositioned, meaning the defect is within the unit itself. Took it to the dealership and both the service advisor and technician confirmed the issue. They will replace the HUD. The part has approximately a one month lead time.
Interesting, same spot showed up in the HUD of my Acura Integra. Like you, the unit had to be replaced. They have to take off the steering wheel, which was not at all centered when they returned the car to me. The Acura service experience is certainly nothing like Lexus, so I'd imagine you won't have a similar issue.