I remember now. Back in 2018 when I first bought the bike, I noticed that the two teeth on the two halves of the hinge plates do not actually contact when the frame is closed. Instead the center slot is the only point of contact with the 13 mm block. So when the latch is tightened then there's about a 0.5 mm gap between the two teeth allowing the hinge to rotate and flex. What I did was I filed down the inside faces of the teeth to make them as parallel as possible and then shimmed them so that they almost touched when the latch was closed. This greatly reduced the flex.

Last year just before I started the tour in Japan, one of the shims fell out and was lost, so I removed the other one too. it was during the middle of the ride that the problem occurred with the hinge starting to develop play. So I guess for the 6 years before that, the shims prevented this from happening.

I had forgotten the importance of why I put the shims in there, otherwise I would have immediately replaced the one that I lost.

I'll try measuring the gap again to shim the gap between the teeth, and then maybe on the pin edge I will add some aluminum tape to try spacing out the runout that has developed.