Can't seem to find something similar so I am going to ask a question.
I recently bought a vintage(mid or late 80s) steel road bike and the back wheel seems to be moving off center and towards the left side of the rear triangle bottom part. The wheel is a tiny bit untrue but nothing out of the ordinary so I don't think that this is the issue. If I take the wheel off and put it back on again correctly it just moves a little bit to the left side after 10-20km. Any ideas on why this might be happening? First time I noticed it was because the tyre started rubbing on the frame and I don't want this to happen anymore.