This particular issues is an OLD favourite with Google Maps and Tabs and is visible in your demo as well
j-cook.pro/index.php/demo/cities
Click any city, click the map tab
Grey empty box..
This is due to the tab content needing a refresh and normally you would just reload your map using an onClick / onLoad type event and a custom function that just requires the data from google.. but of course we are using a fly form rendering. If you are planning on using TABS as an option in your builder, you need an integrated fix for this as any tab (except the FIRST loaded tab) that has a map in it will always be empty and grey
If anyone already has a work around for hidden tab content + J-Cook map, please share your fix.
Thanks