Is easy to replicate the problem.
Just do the same thing, don't put the shifter all the way up to the 3rd gear position and release the clutch.
The problem isn't that you are driving and then it pop out alone. The problem is that
sometimes when you shift into third something gets in your way stopping you to push all the way up to the 3rd gear position and when you release the clutch you will hear the grinding and the shifter gets into the neutral position.
The workaround that I found to this issue is to be aware when the 3rd gear is not in the correct position and when I notice something weird that doesn't let me shift into 3rd effortlessly I just push a little bit harder to force it until it gets to the desired position of 3rd gear.
Is a bit difficult to explain it but at least for me has worked so far.
I hope that some of you can notice this the same way I do and use this advice in a positive way.
