Why is safari flashing up a fixed element at a random point on the page?

My hero interaction involves a ‘browser sized card’ scaling down into a PC monitor, which is fixed at the bottom of the screen. I’ve made every section afterwards relative with 99 z index, so as to hide this element for the rest of the site (I was under the impression this was the most effective way to do this, but open to advice here!). On Chrome it seems to work fine, but i’ve noticed on Safari the monitor + gradient flash up for a few seconds just after the video embed (and flash up again if i scroll up past the same point). I’m completely baffled as to why - any help greatly appreciated.

My read only is - Webflow - PTYA

(note this interaction is hidden on mobile; it was only meant to be a desktop sort of ‘easter egg’)