Yes - it seems like microscopic invisible amounts of grit can cause those balls to jam. And when they do, they are hard to pop out and clean. That's the main thing holding it in a gear - there are little tabs on the spring loaded dogs that the collars go over, but I think those mainly hold them very lightly in neutral, and don't have the ability to do much of anything against the leverage of the shift lever.

Popping out of gear isn't really related to the syncros - by the time it is in a gear the syncros do nothing functional - all they do is hinder the engagement of a gear while a speed differential exists. No effect on anything once in gear. Often, if the gearbox insists on popping out of gear when you speed up and then deccelerate it just means there is some slack in the bearings.
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I'm JohnMc, and I approved this message.