Siteground offers an option to add the Maps API Javascript library URL to the render blocking exclusion list in the SG Optimizer plugin.

If you are seeing broken maps for logged out users, but everything looks fine when you are logged in, then you can use this as a solution. 

If you need help to configure this please contact Siteground support. Their optimizer plugin is constantly under development and they can provide you with the latest tips and tricks related to configuration.

Some information about the feature is available here to help you get started: 

https://www.siteground.com/tutorials/wordpress/sg-optimizer/frontend-optimization/

You can find the URL in the code source of the any page that shows a map, and you should get this from a page with a working map, which may mean you need to be logged in to adjust the setting. The URL will be something like:

https://maps.google.com/maps/api/