To check that the focusing helical is correctly assembled, see that the focusing cam protrudes exactly 7.50mm from the flat mounting face of the lens mount (the part that snugs against the flange on the camera.) when set at infinity. If that's not the case, the helical is probably assembled on the wrong start.
Also check that your LTM adapter is exactly 1mm thick. As in 1.00mm, to 3 significant digits.
If neither of these are the problem, this lens must have been built with an infinity shim ring inside like other Canon LTM lenses, and some nincompoop took the lens apart and lost the ring.
Well, the other possibility is someone took the glass out and put it together wrong...