wrong position of guitar string number

Try this. If it is as you would like, I set myself as follows: I selected all the elements and disabled Automatic Positioning, then Ctrl+R

The reason the 3 is out of position is that it if it were in its default position, it would actually overlaps very slightly with the preceding 1. So really, the simplest solution here is just move the 1 every so slightly to the left, or the 3 ever so slightly to the right, or both. Or, slightly increase the minimum note distance setting in Format / Style / Measure so the notes never get so close that this collision is an issue.

If you do wish to move the string number below its current position, MuseScore should automatically update the "minimum distance" setting along with the offset, to allow for the "collision" (you're actually moving it so far under the 1 there is no more collision, but MuseScore doesn't really know that). Normally the updating of the minimum distance settings happens automatically, but there are a few cases where it doesn't, and manually adjusting the offset of a finger via keyboard shortcuts is occasionally one of them. So try adjusting it one notch too far, then back up a notch. Then the minimum distance will be updated correctly, and it "sticks".