Just tested with an iPhone 8 Plus and iPhone 13 Pro. I can reproduce this in all devices. You need to scroll up and down sometimes fast and sometimes slower. Try at least 10 times and you will see it.
This has been here a while, just has to do with subpixels and how fixed position works on various device screens. Position sticky, I wonder, might have a few rendering quirks in iOS safari, Ive noticed as well.
Likely best to ignore as I think XenForo is simply using this API and while they could hack it by offsetting 0.5px/1px or by adding a bg behind it, eh.