GoogleMap.OnMapLoadedCallback is called as soon as the style is loaded in GoogleMap.setOnMapLoadedCallback. Per docs, it should only be called once rendering is entirely finished:

Sets a callback that's invoked when this map has finished rendering. The callback will only be invoked once.

If this method is called when the map is fully rendered, the callback will be invoked immediately. This event will not fire if the map never loads due to connectivity issues, or if the map is continuously changing and never completes loading due to the user constantly interacting with the map.

This is unfortunately not trivial to implement. Causes signalapp/Signal-Android#12723 (comment).