Nikkor AI-S 50 1.2 backlash

I've bought an used Nikkor AI-S 50 1.2 on Ebay. The lens I've always wanted to own.

It arrived and it is in excellent condition (clean glass, cosmetically excellent) and takes great pictures. However, I've noticed that there is some small slackness (about 0.5mm) in the lens. It looks like that there is a very small frame shift in the viewfinder when I change the direction of turning the focus ring. Perhaps something is not properly tightened?

It has no impact on image quality or focus accuracy, there is just that very small frame shift.

I wonder whether this is normal for these AI-S lenses. This is my only manual lens.

Would you keep or return such lens?