Any command including the attribute "y=0" "y=1" or "y=2" might not return correctly/detect players.

Steps to Reproduce:

1. Go to y-level 1 in a world (picture 1)
2. Type a command that includes the attribute "y=1" and for simplicity "dy=0", e.g.: /testfor @a[x=-5000,dx=10000,y=1,dy=0,z=-10000,dz=20000]
   (make sure you are not outside the specified x and z range!) (picture 2)
3. Notice how the game outputs that no targets match the selector even though it should have listed you. (picture 3)
4. You can repeat the same for y-level 2 and a command with "y=2"

Observed Results:

A command with "y=0" does detect you when you are standing on y-level 0 and 1.

A command with "y=1" does not detect you when you are standing on y-level 1.

A command with "y=1" does detect you when you are standing on y-level 2.

A command with "y=2" does not detect you when you are standing on y-level 2.

A command with "y=2" does detect you when you are flying close above y-level 2. (Position still displayed as 2)

Expected Results:

The commands should only detect the player when the attribute specifies the same y-value as the position of the player in the world. 
(examples with "dy=0" for simplicity reasons)

It works correctly for any other y-level than the mentioned. e.g.:
The command /testfor @a[x=-5000,dx=10000,y=3,dy=0,z=-10000,dz=20000] will only detect players on y-level 3 which is correct functionality.