How to reproduce:
1. Open the attached 'Animation-bug.zip' project
2. Open 'SampleScene' Scene
3. Open 'HexGird' VFX Graph and observe that hex particle ping-pongs in the Game View
4. Notice that Sample Curve node is set to Ping-Pong
5. Connect Random Number node's r port to the Add(float) B port
6. Notice that hex particle animation is not looping in the Game View

Expected result: Hex particle animation loops according to the Ping-Pong curve
Actual result: Hex particle animates only once and then stops

Reproducible with: 2019.2.20f1(6.9.0, 6.9.2), 2019.3.0f6(7.1.7, 7.1.8), 2020.1.0a22(7.1.7, 7.1.8)
Could not test with: 2017.4.36f1 HDRP is not introduced, 2018.4.17f1 due serialization errors

Notes:
- Video of reproduction is attached in Edit
- Reproducible on macOS
- Animation works fine when the Curve is set to Loop or Clamp and Random Number node for time interpolation is used