Hi Daan,
I rolled back the plugin to a version 3.xxx and it worked fine. Just updated to the latest version again and still had the same issue.
I deactivated WP Rocket & cleared cache in case that was an issue, and switched the Optimization mode to ‘Automatic’ and navigated to a few pages with the ‘Montserrat’ font on them eg. https://wildambience.com/wildlife-sounds/australian-boobook , as well as adding the string ?omgf_optimize=2931f to the url. But still only picking up ‘Opens Sans’ & ‘Hind’.
However, I then discovered the Autoptimize plugin had the setting ‘Remove Google Fonts’ selected (it was set like this previously for some reason and worked ok with the old version of OMGF) and once I switched this back to the default ‘Leave As Is’ setting I think this has fixed the issue. I had to refresh the OMGF setting page a few times but after about the third refresh it has picked up Montserrat font. All good to go now! Thanks for your help Daan. It might be worth noting this issue for other users who use the Autoptimize plugin.