Snap issue: GTK rendering emoji tiny

I have this bizarre issue with one of my snap packages I maintain. It’s about the GNUnet Messenger application called messenger-gtk. It’s an app to chat with other people, exchange files and such. It allows to write emoji within text messages via its own emoji picker implementation which essentially uses a list of buttons with labels (in code you will find calls of gtk_button_new_with_label which means it’s a GtkLabel inside a GtkButton, nothing special).

However while it sounds simple. I get following issue that all emoji are rendered tiny compared to the expected size while alphanumeric characters (I assume all ASCII characters) are rendered as expected.

Also this issue only applies to the snap package. It does not happen with the flatpak of the same application. It does not happen when building and installing natively on any system I’ve tested on.

Here’s a screenshot of how it looks:

Here’s how I expect it to look (ignoring the theme difference):

Any ideas what’s causing this? Otherwise the application runs without issues. I can also click on the emoji buttons to add them to messages. However they will not render properly in the text boxes either.