.
...rarely, but still sometimes, a bicycle frame gets constructed with seat stays that are either uneven in length, or warped.
This results in a frame where one dropout is slightly higher than the other one. So when you insert the rear wheel, it tilts a little bit.

This can cause the problem you are observing. Have you actually measured both seat stays for length and straightness ?

It's not always noticeable to the naked eye, and it doesn't take much off level in the dropouts to cause the wheel to tilt. It's not something I would expect to see in Japanese built frame, but it does happen. Sometimes you can find one spot where the effect is minimized, by sliding your rear wheel back and forth in the horizontal dropouts. If your frame has vertical dropouts, and this is the problem, you're kind of stuck with it. It's not an easy problem to correct, if that's what it is.