UI element (menu button) weirdly visible at first load on Desktop/mobile

Hello,

I experience an unwanted menu button appearing at first loading of a website.
Cache clear needed to see it.
It comes from wp-content/generatepress/mobile.min.css menu toggle "system", I was able to see it by making mobile.min.css active

Here is the ugly unwanted button appearing at first launch, so bad first user experience.
https://www.dropbox.com/s/747y6m5o4ol9gdu/gpissue.png?dl=0

So here I'm facing 2 issues:
(1) on desktop the uggly button is visible few time, it shouldn't
(2) on mobile the uggly button is visible few time before being changed by the good one (3)

Thanks for support