Must be the firmware. I have this lens and the 33, which are great btw, and had a problem with the latter.
I tried to update to the latest version, the problem is there are two, a white silk screen and a golden something.
Once the updated was done, the lens started to focus continously.
I deleted all the updates and installed one of the two, I don't remember which one.
Then the lens worked perfectly.
The problem is the Viltrox customer care doesn't exist. I tried to contact them to know the difference between the two updates, but nobody ever replied.