Strange image source in case of a font-awesome icon

Hello,
in case of a font-awesome icon on a new admin page a strange image src-attribute will be rendered:
<img src="font-awesome" alt="">

Steps to reproduce:
1. add a new admin page
2. select a font-awesome icon for the admin page and save
4. check console (browser dev tools) -> an 404-error GET https://<url>/wp-admin/font-awesome appears
5. search for img[src=”font-awesome”] on inspect mode (browser dev tools) -> you will find <img src="font-awesome" alt="">

The page I need help with: [log in to see the link]