That phenomenon is caused by a so called FOUC. You have some sort of a conditional asynchronous CSS loading enabled – I guess via cache or WP Rocket or something else? That is a great feature, but without professional setup for vital inline CSS can lead to unstyled content flashes.
If you disable that feature (asynchronous CSS or some similar name), it will resolve these issues.
Best, Ernest Marcinko If you like my products, don't forget to rate them on codecanyon :)