To reproduce:
1) Create a new project
2) Install Probuilder package
3) Create a new shape (the default cube will work)
4) Select one of the modify tools from the toolbar at the top of scene view (personally tested the surface one)
5) Select one of modifying points (a surface, for example)
6) Hold Shift and drag one of arrows

Expected: Probuilder's Extrude is performed
Actual: Default mesh extend/smallen is done instead

Reproduced in 2018.3.0b8, 2018.3.3f1
Not reproduced in 2018.3.0b5, 2018.3.0b2, 2018.2.20f1

Not reproduced in Win 10