Alfresco can automatically created a new version on property updates, not sure how the cloud version is configured. Default is to auto-version on property updates. So it may well be that the content isn't updated, only properties, but the version number still increases. If you download the file, is it then the latest content version? Does it then differ to what is shown in preview? The preview content file is loaded via a url, it does have url properties to make sure it doesn't load a cached copy instead of loading the latest from server. Possibly you are using a proxy that does cache even if it should not?
If you manually refresh your cache, does i load the new version of the preview then?
Reading the latest it mentions something about latency in generating previews in cloud. Maybe someone from Alfresco with detailed knowledge of cloud can fill in here. For all my on-premise installations (I manage around 10), this is not a problem.