Steps to reproduce

1. Open attached project
2. Open scene "test"
3. Select "Particle System"
4. Open animation window
5. Open Curves tab
6. Select two keys of position.z that form a peak
7. Grab the bottom of the selection box and drag it up to flip the curve
8. Notice that once Y axis values of keys are close the curve starts to flip more than once

Doesn't reproduce when scaling using top, left or right sides/handles.

Reproduced with: 2017.3.0b5