It seems that the defintion of EPSG:2169 is not valid on 6.3.1, I use Mapserver 7, with proj4, the reprojection of EPSG:2169 to EPSG:3857 works perfectly, but with proj 6.3.1, there is a delta of 10 meters on results.

Here is the definition in proj 4 :

+proj=tmerc +lat_0=49.83333333333334 +lon_0=6.166666666666667 +k=1 +x_0=80000 +y_0=100000 +ellps=intl +towgs84=-189.681,18.3463,-42.7695,-0.33746,-3.09264,2.53861,0.4598 +units=m +no_defs

Please add a test case with input and output coordinates and a reference about what is correct. Current Proj version is 8, have a try with that as well.

not reproducible