Reproduction steps:
1. Open the attached project "Repro.zip"
2. Open scene "SortingTest"
3. Notice that CanvasFRONT has white image while CanvasBACK has brown and black
4. Select EventSystem and Play the game
5. Hover the cursor over the spot where white and black images intersect and notice the EventSystem log

Workaround: setting Front canvas in higher layer fixes the problem

Actual result: EventSystem returns pointerEnter the image which is in the back canvas, even though there is an image in front of it
Expected result: EventSystem pointerEnter is expected to return object which is in front

Reproduced on: 5.1.4f1, 5.2.4f1, 5.3.5p1, 5.4.0b19