The answer is simple, this is not a bug: Figma uses a fixed set of emoji (they have a custom renderer so they can’t use your system emoji, they have to implement them manually) and this set is a bit outdated, so some emoji like these circles are not included in this set, Figma simply doesn’t know how to draw them. They implemented emoji just a couple of years ago, as far as I remember, before that they didn’t work at all.
Does Figma intend to update the emoji list or is there something else planned? Is there an easier workaround than adding a colored ellipse with the text and grouping them with auto-layout?