That's ridiculous. How long does it have to be in reverse? Longer than it takes to back up preparing to get out of a parallel parking spot?
As a software engineer, I can easily imagine a scenario where, when the car were put into reverse, it initiates a command to the unit that controls the mirror tilt, but is put back into drive before the mirror tilt completes, resulting in the command to restore the mirror position getting ignored or lost. The software designer could have easily, reasonably or not, assumed no one would go into reverse for less than 2 or 3 seconds.
Now, as human being who is very detail-oriented and experienced with software and device control, and who has learned that most other people are not nearly so, I can easily imagine a scenario where someone who works in service hearing someone tell them about how "if you go from reverse to drive fast [the mirror] won't go back up" and without understanding the details, parrots that explanation to a customer because, in their own mind, it seems relevant, even if it doesn't fit the context of the customer raising the complaint.
Remember that Tesla, like any other company, is populated by people who have limited understanding (to varying degrees) of all of the issues involved and attempt to explain something that made sense in their own mind, even if it is obviously wrong to anyone who puts thought into it. Even purportedly smart people believe some pretty absurd stuff at times (e.g. moon-landing is a hoax, etc.).
Not everyone is a critical, evidence-based thinker; in fact, so few are that those of us who are often have trouble in general society. I remember meeting with my daughter's teacher during a parent-teacher meeting, I pointed out some error on a sheet presented to me that, at least in my mind, mattered. Her response, delivered in a condescending tone, was, "Oh, you must be an engineer." Catagorizing me as picky, detail-oriented person was her defense against her perceived threat to her sense of self worth for having made a mistake, instead of just saying, "Oh, thank you for bringing that to my attention." Sadly, her response was the rule, rather than the exception.