I resolved this via e-mail thanks to the Gibbon support team; but they kindly asked me to post it here for other people to see in case they experience the same issue.
When performing changes over .twig.html files you have to clear the framework cache by going to Admin > System Admin > System Check > System Data > Clear Cache.
Remember that changes done to these files will be overwriten when Gibbon is updated to a later version.
Thanks for sharing your solution! If you change your System Settings > Install Type to “Development” then it will automatically recalculate the cache anytime you change the template files. This helps if you’re making a lot of changes, particularly on a local install.
Rather than editing the resources/templates, you can also place a copy of any templates you change into a themes/Default/templates folder. The system will use these first, then fall-back to the ones in resources/templates, allowing you to selectively override templates without them being replaced on the next system update.