How to reproduce:
1. Open the attached project
2. Open Test scene
3. Select AnimaControll in hierarchy and open animator window
4. Select transition between Anima01 and Anima02
5. Edit Settings: set Transition Duration to 0 and Exit Time something in a range [1e-04; 9.9e-12]

Actual: transition preview completely disappears, changing setting doesn't make it reappear
Expected: transition preview looks same as with settings: Transition Duration 0, Exit Time 0

Workaround: After it disappears Undo the last step and select transition again from animator window. Or recreate transition

Not reproducible: 5.2.4p1