To reproduce:
1. Open attached project. It is the Unity UI sample project.
2. Run the Menu3D scene
3. Press the 'Settings' button. Notice that the panel disappears for a split second.
4. Press one of the buttons inside the settings tab, notice that a new tab on the left appears for a split second before animating into existence

It's as if the last frame of an animation gets played before the rest of the animation.
Regression since 5.2.0f1