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The reverse gear has a bronze blocking ring and doesn't do a very good job matching the speed before being engaged. If you don't come to a full stop and try to shift it into reverse sometimes it grinds a little. Well, that over time leads to rounded engagement teeth. It's gets worse from there.
To answer your question, you most likely have a broken plastic fork pad that is not engaging the synchro into gear all the way. This in turn, "pops" out of gear and will not stay in.
The first 2 pics shows the rounded off teeth. The third pic shows broken fork pad still installed in the synchro.