How to reproduce:
1. Download attached project file
2. Create a new GameObject and add an "AnimationPlayer" script to it.
3. Turn this new GameObject into a prefab by dragging it into the assets view.
4. Select Prefab in Hierarchy window and press the "Add 2D Blend Tree" button in Inspector (Important! Don't apply changes after adding the 2D blend tree.)
5. Enter Play Mode

Actual result: When a MonoBehaviour is an instance of a Prefab, Undo.RecordObject fails to properly record some details when adding objects to a list, causing data to be lost when entering Play Mode or leaving the scene. The same changes will be applied properly if the MonoBehaviour is not a part of a prefab.

In technical detail; if the MonoBehaviour is a Prefab, an addition to a List<Foo> where Foo is a serializable class causes the prefab override to store the new List size, but no information about the new element.

Reproduced with: 2017.1.2f1, 2017.2.0f3, 2017.3.0b3