Reproduction steps:
1. Open the "AnimatorWeightedCurve.zip" project
2. Click on "NewAnimation" in the Project window
3. Change the avatar of the animation to "Unity Model"
4. Play the frames 19 to 21 slowly
5. Observe the Animation Window

Expected Behavior: The object moves steadily in the Z direction as recorded in the animation
Actual Behavior: The object moves far in the Z direction and then comes back on the next frame

Reproducible with: 2018.3.5f1, 2019.1.0b2, 2019.2.0a4

Note: Could not reproduce on 2017.4 as the Weighted Tangents were introduced in the later streams. If you uncheck Weighted Tangent on the frame then you will see that the Object comes back to the normal position. The issue does not seem to reproduce after unchecking Weighted Tangent on the very first frame. The affected frame in the attached project is 20th.