Reproduction steps:
1. Open project attached (Unity3dRaycasterBug.zip).
2. Open scene "RaycasterBug".
3. In Hierarchy select EventSystem.
4. Press play.
5. Move mouse over game while checking EventSystem information.

Expected result: Every position on screen should report Text or Sprite.
Actual result: There is an area below the Sprite that reports nothing, that is EventSystem considers those positions as empty

Reproduced on: 5.3.4f1, 5.4.0f3, 5.5.0p4, 5.6.0b4.
Not reproducible on: 5.3.0f4, 5.3.3f1.
Regression introduced in 5.3.4f1 (or in 5.3.3 patches).

Note: There is a Sprite in the middle, in front of text. Text covers all the screen, so raycast should always return Sprite or Text at any time.
Before 5.3.4f1, The area below Sprite (where we do not get raycast value) used to return Sprite information.