Just checked and mine is missing that bolt too, I’ll get one ordered.
On the subject of stopping the bouncing, I’d be interested to know what others have tried as I did some testing a while back with my gx AXS and gx cable mechs, and determined that the clutches are about the same strength – it’s the absence of the shift cable providing tension, and the extra weight of the mech which allows the mech body to bounce freely once any chain tension is lost.
Given that the only reason the mech needs to rotate on the main pivot bolt is to allow for rear wheel installation – when shifting all chain tension/adjustment is done by the cage – I came to the conclusion that there just needed to be a bit of extra ‘friction’ between the main bolt and the mech body to stop is dangling in the breeze like a pair of 90 year old’s bollocks! 😁