To reproduce:

1. Open the project, attached by the user (SetPivotTest.zip)
2. Open the "Main" scene
3. Enter Play mode
4. Left-click several times in the game (this changes the position and the pivot of the white square)
5. Observe how white square is hovering around the point in which it initially was instead of jumping to the red target

Expected: the position change is not ignored when the pivot is changed in the same frame as well. In this case, the white square jumps to the red square's position and has an offset based on what the pivot point values are.

Reproduced in 2017.3.1p1, 2018.1.0b6, 2018.1.0b7, 2018.2.0a1
Did not reproduce in 2017.2.1p4, 2017.3.1f1
Regression introduced in 2017.3.1p1

After calling rectTransform.position = Target.position; you must call rectTransform.ForceUpdateRectTransforms(). Other option is to use the proper .anchoredPosition call which will work properly.