I am using the current version of Numbers, but maybe in your version once you select "…Fit to Content" you may have to select it again to deselect it. This is often the case with menu items. Select them, the item shows a check mark next to it, select it again and the check mark goes away (deselected).

On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.