Reproduction steps:
1. Create new project.
2. Create Particle System.
3. Turn on its Size by Speed.
4. Select Size Curve.
5. Press Undo.

Expected result: Particle System Curves will be turned off and Size by Speed will be unselected.
Actual result: Size by Speed will be unselected.

Reproduced on: 5.3.7p2, 5.4.3p4, 5.5.0p1, 5.6.0b1

Note: You can also edit selected curve even if Size by Speed is unselected. The same issue and with other Particle System Curves.