I know smooth shift cannot reduce the scale once extended, (although it has the numeric value to do so.
What you can do to extend selected poly’s along there normal and maintain the poly’s scale is to
0. switch to ‘selection’ in ‘Modes’ tab
1. select the desired poly’s.
2. use smooth shift (click once) but do-not extend,
3. switch to Scale tool then switch back to smooth-shift and extend.

i believe what happening is when using smooth-shift it scales along the poly’s normal with the entire object as some kinda base for scaling.
when switching to scale then switching back to smooth-shift, the function see only the polies selected and there normals without any basis for scaling. so it extends without any scaling.