Hi Daniel, this issue has been there since WordPress 4.7 because since then WordPress does not insert the image Title field along with the image link/tag when embedding new images or image galleries.
Sadly, FancyBox has no way of knowing the image title if it is not there in the page source as link title or image tag alt attribute.
There are two ways to get around this:
1. The official SEO optimal WordPress way: give your images an alt attribute by copying the image Title to the Alt field. This will both be good for SEO purposes and for FancyBox to know what to show as title.
This might mean you need to go though a large amount of images if you use the WordPress Gallery function a lot on your site. In that case, consider to:
2. Install the plugin https://wordpress.org/plugins/restore-image-title/ created to restore the old way of embedding images. This will allow you to keep working the same as before.