Customizer Font Changes Not Matching Result

Thanks for your help. I'm going to go without that plugin for now.

Try using the suggested settings here:
https://generatepress.com/fastest-wordpress-theme/

It turns out it's the "Optimize CSS Code?" toggle.

Uncheck it fixed the issue? That's very weird - definitely haven't had this reported before.

If you want to dig a little further, try to deactivate all other plugins and only keep Autopimize activated to see if the issue still exists :)

I'll do some testing on my end as well.