I bought the RS3 Pro full combo kit about 8 months ago along with the Tethered Control Handle and a bunch of other accessories. Everything still works great except the Tethered Control Handle. I have numerous problems with it:

• The main trigger switch sticks in the pushed in position and won't pop back out so the gimbal remains stuck in lock axis position. The trigger button eventually pops out of it's own accord
• The joystick intermittently won't work so you have to retake the shot leading to wasted time on set.
• The record button misfires intermittently so when you press it, it does not start or stop recording which leads to lots of missed takes, and is a major PITA when you are filming events and one take scenes.
  

These problems make this $400AUD control grip absolutely useless. I honestly expected better quality for such an expensive device.

Note that I also own the Tilta Ring Rig and both of their remote control handles work flawlessly without any problems. If Tilta can acomplish such good quality why can't DJI?

My advice to any professional shooter considering buying this tethered control handle is don't! The quality is just not there and definitely not worth the high price tag

There is a chance that the handle has some contamination. I have repaired a few items that had sticky buttons or intermittent buttons/functions.

Get a really good spray contact cleaners.
Use it generously on the bad button and while it is still wet vacuum the area with a small detailing nozzle.

it's a great way to deep clean switches.

Not sure if this will help you, but it might be worth a go.

Does the "Gimbal Lock" work when you hold the triggered on the tethered controller ? I have issues with mine.

I recently purchased the tethered control handle, but happy to say I don't have any issues after a few weeks of use.  It sounds like maybe a pin got bent in the small DIN plug or is perhaps loose?  Not sure why they go with that design plug/cable, but I guess they wanted to keep the size down?