Change 326526 had a related patch set uploaded (by Bmansurov):
Remove redundant "prefix" from ResourceLoaderImageModule modules

https://gerrit.wikimedia.org/r/326526

A few observations:

• Icons are present at Special:MobileMenu: https://en.m.wikipedia.org/wiki/Special:MobileMenu
• Icons are present when debug=true: https://en.m.wikipedia.org/wiki/Main_Page?debug=true
• Icons are present on the staging server: http://reading-web-staging.wmflabs.org/w/index.php?title=Main_Page
• Icons are present on the beta cluster: https://en.m.wikipedia.beta.wmflabs.org/wiki/Main_Page

So, the issue is with the production environment whether logged in or out.

I've submitted a patch above which cleans up the icon code, but I don't know if it will fix the issue as the issue is happening in production only.

Pinging, Release-Engineering-Team in case they can help.

I couldn't replicate this on https://en.m.wikipedia.beta.wmflabs.org/wiki/Main_Page as you state.
Whenever I see this, my thoughts immediately turn to MediaWiki:Mobile.css
I ported MediaWiki:Mobile.css to the beta cluster - and of course after doing this the beta cluster was showing the same problem
Bingo: https://en.m.wikipedia.org/wiki/Special:MobileDiff/754478793

Worth an upstream bug @Krinkle ? Seems we could/should lint this css?

Good job, @Jdlrobson.