To reproduce:
1. Create a new project
2. Create a Cube in the Hierarchy
3. Switch to Local axis
4. Set the Cubes transform rotation Z value to anything but 0
5. Rotate by the X axis and the Y axis

Expected result: the Cube rotates exactly by its local axis X or Y
Actual result: rotating on X and Y axis changes all transform rotation values

Reproduced on:
2017.4.0f1, 2017.4.23f1, 2018.3.8f1, 2019.1.0b6, 2019.2.0a8

Notes:
The game object always rotates exactly by its Z value.