When you are in the F-Curve, while one keyframe is selected, you could just press the s key, and it should be much finer. Go back to all keys with the h key. Besides, holding down the 2 key should help to zoom in or out where the “mouse” indicator is.
If the “effect” consists, please check the Timeline Menu> Edit> Snap Settings, and check if the F-Curve: Y snapping is on; if so, set it to off.
Besides, the value of a keyframe is given in the Attribute Manager. With the alt/option key down, you can “dial” in small values quickly.
To avoid endless guessing, please make a copy of the file, delete everything but the camera and share it here; I will look.