You write you did it for every viewport. If this is the case then it should be for every viewport.

Maybe it is a matter of refreshing the browser on your mobile device?

Could you supply a link for the site?

Have you tried to simulate a mobile device in your desktop-browser - and what is the result there?

Yes, i've tryed to emulate the "phone view" from my pc, and the sticky bar is all right. 

The link to the site is http://www.gabrieledifazio.com

I've also tryed to refresh the page on my mobile phone ( I've also tryed other phones, but the problem won't go away ), but nothing happend...

It seems that you have forgotten the smallest viewport.

When I emulate a Samsung Galaxy in portrait then the background seems to be transparent - but whenI rotate it to landscape it is OK.

Could you try to check the smallest viewport.

If I use the Firefox browser and makes the window smaller and smaller, then it gets transparent when I reach the minimum size.

This also indicates that something is not OK for the smallest viewport.

Nice site by the way. It seems you like the dark - both for the theme and your photos.

Thx! I'll try, but the smallest view is like the others

Try a preview while pressing Ctrl - and then make a full upload of the project.

If it still not functions, then try and see what happens when you make the bar un-sticky for the lowest viewport.

Or try to make another change for that viewport and see the response on that.