I have absolutely no clue whats going on because now I reinstalled almost all modules and now it works fine. I am unable to reproduce the error even with the config I used before.

@ianperrin sorry for the delay!

I tested your approach:
I removed all other modules and reinstalled MMM-Fuel and the icons are showing correctly. It seems like it is a conflict with a different module.
I will add more and more modules and I will tell which one will cause the issue.

Maybe this can help you:
https://forum.magicmirror.builders/topic/10219/fontawesome-with-mm-2-7-1-not-showing-some-icons

I found the solution. It is backwards compatible but some icons are now Pro. There are now alternatives but they use a different class. Example:
CSS-Class before: fa fa-long-arrow-down
CSS-Class now: fas fa-long-arrow-alt-down

Thank you but I already checked this and the modules have this function included. They return an array with fontAwesome and their own css file. I tried to comment out their own css files but this did not help with the missing icons.

Hi,
unfortunately I still have some issues with FontAwesome and its backwords compatibility. Some icons are still not showing like a arraw facing down or a geolocation point. In the new MagicMirror Version there should be a fix for it ( https://github.com/MichMich/MagicMirror/pull/1525 ) but this doesn’t seem to work properly. I am running it on a Raspberry Pi and the Modules MMM-PublicTransportHafas and MMM-Fuel show this issue.
Does anyone have an idea?
Thanks in advance