I’ve noticed a little inconsistency with my header on desktop. For instance when i click around the pages of my site i see the header space increase in height, my logo disappears, while tagline stays the same. This all happens in a split section which is why i couldn’t get a screenshot but it is very noticeable and somewhat annoying.
I don’t know a ton about WP Rocket, but my guess would be that it’s moving CSS to the footer. When that happens, the HTML loads before the CSS, and things will style themselves once the CSS is reached.
I would look for an option in WP Rocket to keep CSS in the <head> of your website.