Hi,
This phenomenon is called FOUC and it happens, whenever the stylesheets are loaded asynchronously. Usually it is caused by custom cache plugin, that minify stylesheets, and have an asynchronous script loading feature.
There is also an async CSS loader feature within the plugin, make sure you have it disabled, although it should not cause this problem. I suggest checking any cache plugin you have, and turning off asynchronous stylesheet loading features via JS, and it will resolve the problem.