Look closely at the thickness of the bottom of the narrow section compared with the top. The bottom is thick and the top is thin. That means the bottom is stiffer than the top. That means for the same force, the bottom will stretch less and the top will stretch more which causes the bending you see.
If you could shift the origin of the infill and move it down slightly so that the top and bottom were of equal thickness, then there would be no bending.
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