Icon hyperlinks no longer visible in R markdown using YAML html_document

I previously had success adding headers and footers to my R markdown HTML files using guidance found here: Pimp my RMD. No changes have been made to my header.html, but now the icon hyperlinks are no longer visible when knit via RStudio. I confirmed that the URL is still valid. Strangely, the hyperlink via my name is still visible and functioning. When the header.html file is opened in Chrome, the hyperlink icons are present and visible. Why would the hyperlink icons no longer be visible in the markdown HTML?

My YAML looks like this:

title: "TITLE"
date: "DD/MM/YYYY"
      before_body: ./header.html
      after_body: ./footer.html
    code_folding: hide
    highlight: zenburn
    self_contained: yes
    theme: darkly

My header.html file looks like this:

<p style="text-align: right;"><a href="mailto:MyEmailAddress">Alex Brasch</a></p>
<!-- Add icon library -->
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.15.1/css/all.min.css">
<!-- Add font awesome icons -->
<p style="text-align: right;">
    <a href="https://www.flo-analytics.com/staff/alex-brasch-2/" class="fas fa-chart-bar"></a>
    <a href="https://www.linkedin.com/in/alex-brasch-b5231743/" class="fab fa-linkedin"></a>
    <a href="https://github.com/a-brasch" class="fab fa-github"></a>

Solution found here:

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