Hi,
This is called the FOUC phenomenon, and it happens when critical assets are delayed from the page load. It usually happens a minify/cache plugin is used and it’s accidentally configured to asynchronously load all javascrip and stylesheets. In case you use anything like that, make sure to turn off async asset loading – those I only recommend using if you know how to extract critical CSS, and that’s not easy to do.
If that does not work or you don’t use anything like that, then can you please change these two options: https://i.imgur.com/d7razwI.png
These should resolve any output buffer related issues causing the FOUC.
