Help! Elements disappearing/reappearing unexpectedly

If anyone can help please, I’d really appreciate it! This site is nearly done but I can’t seem to resolve this issue! Three elements seem to be competing for screen time. When two appear the other disappears and vice versa. You can see in this video that shows the ‘Test Flight’ header disappearing when the Next button and Close icon reappear and vice versa.

Towards the end of the video I took a screenshot and attached it below. Weirdly even though you can seen ‘Test’ showing on the screen when the screenshot is taken, that’s not what the screenshot actually shows. I found this also happened when I tried to do my first screen recording of the problem. So I suspect this may be an issue with webkit or safari after I added a code to the header to stop animations from flickering. I’ve since removed that code but this issue persists.

I’d be super grateful if anyone could tell me what the issue is because I’ve been waiting to fix this before I can push this site live. But I can’t even seem to revert this to its original state before this was an issue so I’m genuinely stuck for a solution.

Here is my site Read-Only: LINK