How to reproduce:

1. Open attached project (reproProj.zip)
2. Open scene Scenes/test.unity
3. Play the scene
4. Open the Animator window
5. Go into the Blend Tree layer
6. Select the Blend Tree
7. In the Inspector, try to modify the current position in Blend Tree Diagram
- Note that it does not do anything
- Using the sliders in the Animator window still works
- Reproduced in Version 5.4.0b6 (2c3d6b93e789), Version 5.3.1f1 (3b3e8231502f)
- Not reproducible in Version 5.2.4f1 (98095704e6fe) (but blend trees are broken in that version)