Compatibility with `posthtml-include`

In base.html the <include> file never gets rendered.

<body>
        <!-- Doesn't work -->
    <include src="partials/header.html"></include>
    <main>
        <block name="content"></block>
    </main>
</body>

However it does get rendered within a layout:

<extends src="base.html">
    <block name="content">
                 <!-- Works -->
        <include src="partials/header.html"></include>
    </block>
</extends>

Any idea what could be causing this issue?

posthtml / posthtml-extend

Compatibility with `posthtml-include` #59