I bought one of these a few months ago, and when I tested it prior to purchase, it also could not focus on infinite. The guy I bought it from was clueless (a second-hand dealer), but the lens was mounted with a 13mm extension tube when I opened the case. Naturally I removed the extension tube when I mounted it. After some frustration not being able to get focus at infinity, I remounted the 13mm extension tube and presto, it focused at infinity. The extension tubes set and several other bits including PK/A-1.4XL rear adapter came thrown in, so I was not concerned about the focus issue.
I have not attempted to resolve the infinity focus issue, but just use it with the extension tube. When using the 1.4X rear adapter, I have to use a longer extension tube. But it all works fine as shown by this moon shot at 700mm.
But you've got me thinking now about whether it can be adjusted..