Normally I would agree 100%.
When you turn it the focus ring changes.
HOWEVER.
I made a test setup with 3 bottles and focused on the center one.
With almost all settings the front bottle was razorsharp and the center one OOF.
Whatever I tried, manual focus or whatever the front one was sharp.
When I found out that all the screws were set correctly I decided to turn that one.
The first impression was indeed as you say it's the focus motor, it fits because the barrel extends when turning, so I gave up.
Placed the lens back and did one last test, I was struck with delight when I saw that now still the front bottle was in focus but on the back.
I decided to try it again and turned it one whole turn more, when connecting again and testing the focus had drifted to the center.
It took me arround 2 hours to get the result I wanted, but this was more because I was trying to get it picture perfect, when you set it one little bit too much it's out of focus again so it's very very precise.
After one hour I had an almost perfect focus but when I tried the 150mm I decided to give it one more try (that one was MUCH sharper), and after another hour I got it app arround the same quality as the 150mm which is sharp without any blurring of the edges.
So it's correct it's the focus drive probarbly and I won't suggest to do it blindly on all your lenses, but when you have a back/front focus problem before dumping the lens or sending it in I would really test this.
