huuduy216 if the rev-manifest.json file exists, Flarum will use the filename listed in that file.
When the cache is cleared or an extension enabled/disabled, the javascript/css files as well as rev-manifest.json files are deleted.
If the rev-manifest.json file doesn't exist, it will be created along with the javascript and css when a visitor requests the page. This should all happen before the page is returned to the user, so unless there are file permission issues on the server the file should exist.
If there's a lot of traffic on the website, there might be a very small risk of race condition where two requests execute in parallel and delete each other files. For high traffic forums like those this extension might help https://discuss.flarum.org/d/23321-cache-assets
If all this happened but you didn't clear the cache or enabled extensions, it's probably another reason but I'm not sure what it could be.
