In rare cases, the zooming is opposite, and the screen is from a camera in fix location relative to the craft, instead of a moveable one, which means the image get strangely streched when zoomed out.
#2 Updated by email@example.com 20 days ago
It is possible that KSP thinks your modifier key (Left Alt on Microsoft Windows) is being held down.
If keys + and − zoom in and out, then Alt-+ and Alt-− increase and decrease the angular field of view.
When my zoom-in key misbehaves like this, I tap Alt and it behaves correctly.