Steps to reproduce:

1. Open attached project
2. Open scene "scene"
3. In hierarchy, expand Canvas and select its child "Animated UI" object
4. In animation window, enable recording
5. In inspector, set Z position to "100"
6. In animation window, notice that Z position at 0:00 keyframe is set to 100
7. Now change keyframe to the one at 1:00 and notice that position was set to 100 as well

Workaround: Add this animation to the object which can have its Z position animated (for example, main camera) and animate Z position. Now animation will animate Z position of RectTransform as well.

Not reproducible: 5.3.4p3
Reproduced with: 5.4.0b15