Unfortunately I am not able to reproduce your issue (see screencast). If you can provide me with some temporary credentials to your site via help@bricksbuilder.io I am happy to have a look at it.
I’m seeing this too and I have regenerated all css.
The section for the header has a dark background colour. I have set the header to be sticky and slide up after 200px with a completely different scrolling background colour. As soon as I start scrolling, the dark background disappears and there is no scrolling background colour displayed. When I inspect the code, I can’t even see the definition of the scrolling background colour.
Thomas is logged into my site looking at this issue and another one. I just checked without clearing the browser cache (I just refreshed the page I was on) and it is now working so I think he has changed something somewhere.
Just tried changing the scrolling background colour again, cleared the history in two browsers, reopened them and scrolling still shows the previous colour.
As another test, I just installed the vivaldi and opera browsers and neither of these show the new scrolling background colour. I can see it in the builder but not on the front end.