Made a video for the issue that wasn't resolved after 2 weeks at the dealer. - https://www.youtube.com/watch?v=AkZq072azPQ
In your video, to me it is clearly a mechanical issue... it is binding and just not returning smoothly. It is hard to see, but you CAN see the throttle turning as it slowly returns and the RPM falls. Stare very closely at the throttle on the handlebar while listening. Remember that with zero load on the engine, it only takes a barely visible amount of tube turn to create a LOT of rpm change. The throttle should snap back cleanly and nearly instantly when you let go of it, no matter how much you turn it. That is certainly not the case in your video.
Now where it lies, that is the trick, because there is a lot of stuff involved in all that linkage. It could be sticky cable, worn spring, worn or sticky throttle tube, kinked cable, improper adjustment between the two cables, throttle grip or tube touching the weight or bike side, something near the throttle butterflies, etc, etc. Being [engine] temperature related would shift focus to everything AFTER the handlebars and closer to the engine. I am surprised that a competent dealer would not be able to find the issue.