The lock nut does more than prevent rotation of the rod end. To assemble anything there has to be a clearance unless you are assembling using an interference fit. The interference process is not usable for a rod end that requires adjustment.
The second function of the lock nut is to draw the male and female threads together and prevent movement and fretting.
If the nut is loose there will be motion between the two threads dependent on the tolerances that the threads have been manufactured too. More than possible that the female could be at maximum and the male at minimum. The further out that the rod end is adjusted the less support there will be and the less resistance to fretting.
If this condition existed for an extended period then the threads will gradually be destroyed and the rod end released. If the tube is aluminium and the rod end steel then this process will gradually speed up. I do not know what the S76 tube is made from but the eye will be steel and as the rod is below the servo a good chance it will be aluminium.
I would conclude that there is a good chance that the rod end was loose for some time leading to excessive thread wear and release of the rod end.