Fixed element backward changes background page

Hi, when I'm achieving an effect to show a fixed element only when you scroll something weird happens.

I have a fixed element placed behind all other objects and on top of the viewpoint. This element will only show up when there's nothing on top of that element anymore.

Here's a working example in the preview on XD: https://recordit.co/OPB3du0BJM 

And here's the outcome when I create a shareable link: https://recordit.co/R4PMnMUdhe

As you can see the white background disappears when the fixed element shows up and is replaced with a gray color. The white background comes from the page itself, there's no extra white shape below. Even when I put a white shape below it all, it doesn't show up, it's still grey.

This also happens with colours in another document, but then it shows up already and all dotted (see attachment).

This way I can't really show it to my customers.

Version 42.1.22 on Windows. Cloud documents.

Thanks in advance.

Dylan