Hi,
I just recognised that when I mask with rectangular mask brush the segments on the edge are not masked completely. That lead to an unexpected offset when extruding the masked segments (see screenshot). Is it possible to mask all segments completely?
If you can hide all but the polygons you want to mask, you can 100% mask all of the visible polygons.
However, that won’t solve your issue there. Masking is subject to a mesh’s topology. If you want to cleanly pull out that cluster of polygons without any poly stretching, you will need to extrude them, not simply move them.