Steps to reproduce:
1. Open the attached project (case-1202437_LowPassFilterCurves.zip)
2. Select "Audio Source" in the Hierarchy
3. Scroll down in the Inspector and take note of the "Reverb Zone Mix" and "Low-Pass" curves
4. Enter Play Mode
5. Observe the change in the curves
6. Use Spacebar to move the last keyframe of each curve in negative X and negative Y direction

Expected result: Both of the curves overlap at Distance = 375 and continue to do so as the last keyframe is moved by repeatedly pressing Spacebar
Actual result: Low-Pass filter's curve's last Keyframe's time parameter is completely ignored and set to 1.0, which causes the entire curve to scale accordingly

Reproduced in: 2017.4.35f1, 2018.4.14f1, 2019.2.16f1 (Low-Pass only), 2019.3.0f2 (Low-Pass only), 2020.1.0a16 (Low-Pass only)

Note: In Unity 2017.4.35f1, 2018.4.14f1, the same behavior is observed for all built-in AudioSource distance function curves (Volume, Spatial Blend, etc.) along with the Low-Pass curve.