Juha's explanation:

Repro:

Adding myScript.Enabled curve via recording system produces:
- isPptrCurve: true
- Valuetype: Boolean

Adding myScript.Enabled curve via CreateDefaultCurve() produces:
- isPptrCurve: false
- Valuetype: Single

Expected:

Both should produce curve binding that is:
- isPptrCurve: false
- Valuetype: Boolean

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Repro:
1. Add an object and an animation to it
2. Add script component to the object
3. Start recording
4. disable the script - the curve is recorded to the animation
5. Go to Add Curve and see its possible to add the curve again - do so and
6. Expand the curve and observe how the window is broken