Unity: set y rotation messed up the object

I created 3 boxes, then align the three to form some sort of a gate, I set the parent to an empty object so that I can easily move it. And I add box collider inside the gate.

When I try to rotate it on Y axis, the boxes deforms

I just want to rotate the Y, not deformed it. Here are the object

Is this related to the transformation I created before? Something that requires to apply the transformation first? like in blender3d?

• It looks like you have some non-uniform scales at multiple levels of hierarchy. Non-uniform scale doesn't stack cleanly with rotation when combining child & parent orientations. Do you have any scaling applied to the parent of Gate Half? – DMGregory Sep 17 '18 at 16:37
• Wow, how did you know ? I have a parent that has non uniform scale. So I changed that. then the rotation works again :) Thanks! – otong Sep 17 '18 at 16:41