Yeah, there's not much I can do there unfortunately (since I'm using SVG with the actual U and S emojis to show the flag). I can't comment on whether it's your config or not, but I've tested the SVGs on iOS and Firefox/Chrome on desktop to make sure they rendered nicely for most people. Sorry you aren't getting a great experience there.
And Chrome on Android, though with a rendering difference on the not-plain-ol'-U and not-plain-ol'-S (in both cases, blue letters rather than white with a blue background).
Here's how it's rendering for me on Firefox: https://pasteboard.co/rjLtqANVQUIJ.png