TL;DR: There was a bug in the production builds, which caused the FOUC. We've fixed it a while ago, and here is the commit diff for it. Update it accordingly.
It is common to face this issue in development builds. To make hot reloading of CSS work, CSS is not extracted in the development build.
During development, after all of the scripts get loaded, CSS is loaded as BLOBs.
For production, though, the CSS is extracted and a .css file is created, which is used in the HTML template. So you wouldn't face this problem in production.