CSS Excludes is the field on the Tuning tab of the Settings page.
In it, you can enter the names of stylesheet files, like fonts.css, each on a line.
This causes the plugin to include those files separately on the page, which allows you to debug, change order of appearance, etc.
Minification doesn’t handle fonts. They are loaded separately when the browser parses the CSS and sees the need for them. When you preload, however, the browser will trust you and fetch the fonts files earlier.
To find out what’s initiating the loading of a font, look at the Network tab of your browser’s code inspector, find the font and check the “initiator”. You’re likely to find that when OMGF is active, Google fonts are requested by advertising scripts, chat widgets, etc, but no longer from your site’s theme.
I hope that’s clear.