What caught my eye was the fact that your group moved but was a nice group. This makes it less likely that the bore is dirty or not broke in as yet. These things typically lead to bigger groups instead of good groups that moved. The same would be the case if your mounts/rings get loose, your group would get sloppy.
A warm barrel could do it if the stock is not bedded properly & pressures change on the barrel as it heats, although it has to be pretty extreme & a obvious flaw on a MZ barrel. Hot weather & a hot barrel can make sabots shoot different, but it would be bigger groups rather than a group that moves & besides, you are using Powerbelts.
True, there can be other variables, but I have had internal scope adjustment changes occur for both windage & elevation when I have only made a change with one turrent. Oldrookie, I have had this happen MANY times in the past, especially with the "bargain" scopes,
but a few times with good scopes as well. So, I would look at he bedding & the scope first, especially the scope.