How to reproduce:
1. Download attached project file and open "scene1" Scene
2. Enter Play Mode and Right Click with mouse on Red Panel (notice Green Panel appears)
3. Click again on Red Panel, but in different position

Expected result: Panel appears in place where mouse was clicked (only in Red Panel zone)
Actual result: First time, that works fine, but if you repeat it, it won't position the green box correctly. If you look in the Scene Window and have the "RightClickMenuPanel' selected, you can see the gameObjects outline is moved, but the actual image doesn't move.

Reproduced with: 2017.1.1p3, 2017.2.0b7, 2017.3.0b2
Not reproduced with: 5.6.3p4, 2017.1.0b8, 2017.2.0b8, 2017.2.0f1

Regression: Since 2017.1.0b9 Till 2017.2.0b8;
Since 2017.3.0a1;

Workaround: put SetActive(true) in front of GetComponent<RectTransform>().position = eventData.position;