Weird border flicker on mobile version of site

Hi! I’m hoping someone can help me figure this out. I’m working on building a website that has an image at the start. For some reason, on mobile only, when I visit the site the top image flashes a thin grey border quickly before loading in.

There’s no borders applied or anything, and I can’t replicate it on my desktop browser / inspector. Any ideas?

This is the live site:

And this is a picture of the border that flashes for a second:

Here is my site Read-Only: [LINK][1]