In the new Blender 2.80 in the “3D View” window (3D Viewport), when scrolling a wheel to zoom or pressing the num pad buttons (1, 3, 7, 9) to center the view to different sides, the projection mode automatically changes between orthographic and perspective.
To disable this automatic mode changing, in the “User Preferences” window – “Navigation”, change the value of the Auto Perspective checkbox:
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