I don't know why this keeps happening every time I import certain field groups from other websites of mine...
But for my future reference, and in case anyone else runs into this issue, here's a little javascript to run in the browser console.
It opens up the "Advanced" tab of each field, on the Edit Field Group page.
jQuery('.og-item__header.og-collapsible__header:not(.og-collapsible--expanded)').trigger('click');
jQuery('.og-item__body.og-collapsible__body .react-tabs__tab-list li:last-child').trigger('click');
Then I can quickly search if "Save field" is checked...saves a lot of time.