If thats the same pose/animation the issue isn’t necessarily the weightpaint but the bones themselves. Make sure the bonebretargeting settings are properly set, it could be its being translated by mistake.
If that doesn’t solve it, then the vericis have too many influences and the proper weight paint is being truncated. You have a max of 8 influences per vert in engine.
the only other alteration occurs when weigit values dip below .01.
The engine truncates those values so you cannot expect a skeletal mesh to be more accurate than a millimiter / blender will obviously not truncate and present a different result.