Is the issue you're having still with the additional node type for questions and answers?
As I mentioned, the class name is different on your forum. I'm guessing the add-on is replacing the default XF class with a different class name. If an add-on comes in and changes classes around on elements it's not something I'm going to support at all. If you choose to use an add-on that changes the default XF classes then you need to apply adjustments as it's unique to the site that runs that add-on.
The left side is your site, Right side is default XF.
Your site has the class icon name = fas fa-question
Default XF setup has the class icon name = fa--xf far fa-question-circle
I've tried all the Font Awesome weight and my CSS works just fine on it.
My CSS I provided above in post #2 should work on any forum NOT running that plugin.
Final notes to clarify:
I will be fixing our node icon CSS to not target the additional node types (QnA/Articles/Suggestions).
I'll make it so if you leave our node icons blank, it won't load the CSS which will inherit the default XF icon setup
I will not be adding in additional classes to our CSS because a plugin decides to change the default class.