If you look down at the mechanism this will make more sense. The micro switch can be adjusted, likely your’s is; loose, out of adjustment, or faulty. Since it beeps once my guess is that it’s just barely out of adjustment, it’s making the switch as you pass the max shift point. Since the mechanism travels just past that point your loosing the switch contact. Think of it as a cam, when the reverse lever is fully engaged it’s slightly past the top of the lobe. Like I said pull the cover off and work the lever and it will make sense.