Steps to reproduce:

1. Download and import attached project.
2. Open scene: main
3. Select obejct in hierarchy: cube
4. In animator window, double click on "new state" to enter its Blend Tree.
5. Select blend tree and in inspector window try to change top bound from 10 to 5. (GIF)

Expected result: Bound changes to 5, and two bottom motion threshold also changes to 5.
Actual result: Bound changes to 5, but there is no motion with threshold of 5 (it is 1.666..), which is incorrect.

Notes: this only happens when we have two motions with the same threshold at top or min bounds.

Reproduced on: 5.4.0b9, 5.3.3p3, 5.2.4f1, 5.1.4f1