Sometimes the portrait in the game just won’t update, no matter what. The portrait is important because people love to commend the best portrait. And commendation is important if you want to unlock Mentor Roulette.
In my example, I’m a Scholar (SCH):

And I have my Scholar (SCH) portrait set. In this case, I have a different outfit – but it does not matter – it can be the same look too:

As you can see, I go into a Duty and I get this. Again, you can have the same outfit and it will still not work. Once you enter the duty, you get this:

Go out, enter Character, and make a new Gear Set:

Then go into Portraits and Right-Click this new class portrait. Import Settings.

Select the other portrait you already set up before:

And now it works!

I tried all the other guides I found on Reddit and Discord, and nothing else worked. Hope this will work for you too!
Feel free to join my stream! ^.^