1px white space appearing between two pages within index on mobile only

Site URL: https://www.weareglitch.com/we-glitch-teach

I've created an Index on version 7.0 using square block colours as media backgrounds for each page within it - which appears to be working fine - except two blocks where there is a 1px white line showing between them on mobile only. I can't work out where this is coming from as there are no borders, the colour I uploaded is solid, I can't see what setting could possibly affect this - and it's not doing this on any other section so appears glitchy?

Try adding to Design > Custom CSS

/* remove white border */
@media screen and (max-width:640px) {
body#collection-6001da65c3c3515b91f0bf43 section {
    margin-top: -1px;
}
}