You mentioned the problem in traffic & dealer unable to duplicate with multiple techs.
Of the 3 HD products and numerous import machines I've had, I can duplicate the same phenomena when coming to a stop without sufficient distance to initiate a ramped downshift between gears.
It happens when I pull in the clutch in higher gears at speed, hit the brakes and rpm drops along with vehicle speed. One usually gets one click down but nothing afterwards if you continue rapid deceleration.
To restore shifter engagement, in quick succession, I partially let out on the clutch then pull the clutch back in, downshift & repeat as necessary to get back to first gear.
You can do this while still slowing or at a complete stop without the need to pull out in a higher gear.
Assuming your downshift incorporates the sequence;
throttle letoff, clutch in, throttle blip, downshift, clutch out, while engine & drivetrain provide braking, repeat.
I have always had flawless downshifting on any make machine if the distance to stop allows the engine & drivetrain to ramp down to first gear with a load on the drive train. While not an expert, I think this leads to synchronization of the drive system?
One may deduce or question; if the gears & drive system synchronize properly on the up shift, what drive components are different for the down shift?
If the components are the same and functioning, then investigate down shifting sequence for clues.
You may also want to investigate the lubrication type (manufacturer) & viscosity of the drive train section responsible for up & down shifting for more clues.
Hope it helps! These machines are still marvels regardless of the quirks!
Disclaimer: As with any HD product your individual results may vary. Clues provided as an aid to troubleshooting problems experienced by this rider over a 45 year riding span.