Reproduction steps:
1. Create new project.
2. Create Particle System.
3. Mark Size by Speed .
4. Open its Size Curve.
5. Create new Key.
6. Press "Edit Key...".
7. Set value to 0.5 and time to 0.5.

Expected result: New key value will be 0.5 and time 0.5. No additional changes.
Actual result: It will delete base key (that is located on time = 0). Key parameters changed as expected.

Reproduced on: 5.5.0b9, 5.5.0p1, 5.6.0b1.
Works fine in: 5.5.0b7.

Note: There is the same issue with other Curves, not only with Size by Speed.