Check the screws linking the gear knuckle and the gear linkage. It has been reported multiple times that these screws like to fly away. There is nothing to stop the screws from unscrewing, and no thread locking from factory. Remove those bolts and use of a thread-locking liquid before putting them back (Loctite or equivalent).
The manual list the torque for those bolts at 10Nm (7.4 lbf ft), and the use of Loctite®243TM or equivalent (Page 333 of the R version of the manual). Do not overtorque “for safety”: the knuckle is aluminium, over torquing will destroy the threads.