Slic3r: 1.3.1-dev-54a31ee
OS: Ubuntu 18.04.4

Load STL
Select model.
Right click - Rotate - Around z axis...
Input 90 - press ok
Model moves 90 degrees counter clock wise as expected.
Input 90 - press ok
BUG Model doesn't move.
Input 0 - press ok
Model doesn't move. (As expected for relative rotations, but not absolute.)
Input -90 - press ok
BUG Model rotates 180 degrees.

Around x and y axis work as expected other than the previously reported bug about rotations of -1.