It looks like a font issue to me, and I do not see any of the images as "wrong".

http://www.unicode.org/emoji/charts/full-emoji-list.html

All 4 characters are above the basic plane.
U+1F600, U+1F601, U+1F602, U+1F923.
That suggests the SVG agents have no problems rendering high plane characters.

In the first picture, the fourth character is a format that is often used for unimplemented characters.

The second picture shows a font that has colored characters instead of the black-and-white characters of the first.

The SVG file specifies font-family="Noto Color Emoji". Noto signifies a Google font.

You should be able to download that font at https://www.google.com/get/noto/

Definitely a font issue, probably not a librsvg issue. Debian doesn't package Noto Color Emoji until Buster, so we'd have to backport (unlikely) or wait for T216815.

Yes this character seems to be a font-issue. However it is strange, why now resvg does not render any characters any more on the same computer (I run in the meantime sudo dnf install google-noto-\*)

@JoKalliauer resvg doesn't support emojis yet.

This has been resolved as part of the Buster upgrade including the correct google-noto fonts.