@Tasnadi-Otto Is the precomp exactly square? If it is, if you add 1 px to the width of the precomp, does that resolve the issue?
There was a bug in older versions of Templater in the way that crop and fill were calculating depending upon whether or not the source footage was portrait, landscape, or square, as well as whether or not the precomp was portrait, square, or landscape, The scaling bug is fixed in 2.9.6. In 2.9.6, it should behave correctly regardless of which orientation the footage or source are, and you also wouldn’t need to add the extra 1px for square comps.