Preview is rendering using a Spine Runtime. It is using AnimationState like it would in your game, so mixing between animations, tracks, etc should be the same. Preview is configured via the editor, so that part does differ quite a bit from exporting to a file and loading the data.
I'm not sure what you are seeing, so it's hard to say why it's happening. Is it possible to send your project files and instructions on how we can see the problems ourselves?
You could export to JSON or binary and run the Skeleton Viewer to see how it behaves using purely the Spine Runtimes, without the Spine editor parts.
Skeleton Viewer