Reproduction steps:
1. Open the attached project "20180131_UIbug.zip";
2. Open the Masterrepo.unity scene;
3. In Scene view, observe the position of the "ShowResults" button;
4. Enter a play mode;
5. Observe the position of the same button.

Expected results: UI button will stay in the same position as it was.
Actual result: UI button starts changing its position everytime you re-enter the play mode.

Reproduced on: 2018.1.0b5, 2018.1.0b6, 2018.2.0a1.
Not reproduced on 2018.1.0b4, 2017.3.1f1, 2017.2.1p3, 2017.1.3p1.