Ok, have to admit my CSS is rusty enough I'm probably not much help, plus I've been super busy so don't have time to dig into code like I used to. 😢

I'm assuming you are referencing physical positions for the given specific avatar,

example:

background-position: -xpx -ypx;
width: XXpx;
height:  XXpx;

but with that are you also using scaling? like:

background-size:  100%

have you checked if the scaling is breaking as the coordinates stop lining up (proportionality being lost)? May want to double check/recalculate positions based on the scaled size.

especially if you don't have consistent sized avatars, that'll throw off super fast.

I'll see if I can give a more honest dig into it tomorrow.