I know I am not the poster, However I still get this issue as well. It seems to always happen if you are using 1 stud increments on unanchored parts, with join surfaces enabled. I cannot tell if it is the welds or dragging parts with the select tool that causes the offset, as at some times it appears it can be both that causes the offset.
It is a bit hard to visually reproduce at first as the offset starts smaller than 0.001 stud increments, studio wont show you movement that small until the offset compounds enough to reach the thousandths place . This offset chains between parts that are welded together, making fixing the offset a nightmare.
There isn’t a guaranteed method of reproducing this issue, but it happens often and it is nearly invisible unless you have a way to see part coordinates under 0.001 studs.
This offset issue has plagued me for years and I have tried nearly everything to avoid it, but it always reappears whenever I try and build something in studio.