1. What happened
Particle System: Box shape with Scale.Z = 0 changes emission direction. And that is a regression from 5.6. There were no 'Scale' prior to 2017.1, but there were sizes XYZ, that are transformed into 'Scale.XYZ' in 2017+. Issue reproducible both with new PS and the one upgraded from 5.6.

2. How we can reproduce it using the example you attached
- See scene 1 > PS
- Change 'Shape'.Scale.Z to greater than 0
>>> The emission direction changes
It is expected to keep emission direction for 0 and greater.