Back tire

It could be a road damaged tire, but when it happens again, I'm thinking there may be some worn suspension parts the inspection mechanic missed the first time. If they find and replace something, you will see a noticeable difference how it handles and tire problems will cease. I had an older car that did that.

Since your wheels are hub centric, your spacers may have worn just enough to introduce a little wobble. Check around on some of the Porsche forums for suggestions on wheel wobble and spacers.

Have your tires balanced on a Road Force tire balancing machine as Porsche's tend to be very sensitive to tire balance issues.