I'm submitting a ... (check one with "x")
[ X] bug report => Search github for a similar issue or PR before submitting
[ ] feature request => Please check if request is not on the roadmap already https://github.com/primefaces/primeng/wiki/Roadmap
[ ] support request => Please do not submit support request here, instead see http://forum.primefaces.org/viewforum.php?f=35
Plunkr Case (Bug Reports)
Please demonstrate your case at stackblitz by using the issue template below. Issues without a test case have much less possibility to be reviewd in detail and assisted.
Icon is broken (6.0.0-beta.1)
https://prime-ng-600beta1.stackblitz.io

Current behavior
Font awesome icons are not showing correctly (I guess font-family: FontAwesome is not applied)
Expected behavior
Icon is OK (6.0.0-alpha.1)
https://prime-ng-600alpha1.stackblitz.io

Did you see 6.0.0-beta.1 changelog?
Since beta.1, way of using fontawesome has changed.
https://www.primefaces.org/primeng-6-0-0-beta-1-released-primeicons/
Thanks for your answer!
Oops, I missed that... (not referenced in https://www.primefaces.org/primeng though, kind of hard to find) 馃槄
Actually, as I didn't see this problem when upgrading to 6.0.0-alpha.1, I was not expecting breaking changes between alpha and beta.
I understand the Change of plan regarding FontAwesome even though I didn't understand why it was not possible to support both font libraries as earlier (without the 'fa' class addition).
By the way, regarding PrimeIcons, this looks odd:

Not a PrimeNG issue.
"Not a PrimeNG issue", well it kind of is, seeing as Prime changed how to use icons the same way (unless Font Awesome did this? Then why would PrimeNG 5 still work?). I'm sure lots of applications temporarily broke because of this change, as using the original way was easy and simple (not to say it's any harder at all of course, but still takes time to search and fix).