See if this helps:  https://wpforo.com/community/faq/problems-with-icons-font-awesome-5/

As stated in the 1.4.6 release announcement, you'll have to review and update your Font Awesome icons. Font Awesome recently updated to version 5.0 and they have caused a bunch of turmoil in many plugins and themes.

You'll need to edit each forum in wpForo settings and change from the old FA naming conventions to the new. If you use the Custom Fields addon, you'll also need to edit all fields that have icons to match the new FA naming convention.

Posted by: gombi1

Hi,

Some icons are not showing anymore after the last update. Any help on this would be great...

 Thanks,

 

Try to put this CSS code in Dashboard > Forums > Settings > Styles > Custom CSS Code textarea. Save it, delete all caches and do Ctrl+F5 on forum front.

.fa-reply:before {
   content: "\f3e5" !important;
}

Thanks to all for the help! I should read release notes before asking questions.  😀