Custom icon not showing up in the navigation bar

[CharlieBros]

Hello guys!

First of all, I just wanna say thank you for the incredible work done with this project, you peeps rock!

Now, with the issue:

I'm currently doing an internal KB for my work, due to this, I have the need to add a few custom icons, for example, the Windows 11 logo.

I followed the custom icon guide and the icons work perfectly as emojis, however, I'm doing an article that requires a custom icon to show up in the navbar.

First, I had an issue with mkdocs not being able to run, but after following the solution in this issue #3629 I was able to run mkdocs again (BTW, the solution in this issue is not mentioned in the custom icons tutorial section in the Material KB)

However, when I type this in the frontmatter:

---
icon: kb/popup
---

It shows up like this:

If I change the icon to one that comes by default, it works perfectly, like this:

The icon loads correctly when used as an emoji, what gives?