There were some small differences between the Template node and the Input node.
So after a heavy battle with the "Style" tabsheet in my Chrome developer tools, I updated the Template node code to this:
<md-card class="nr-dashboard-textinput" style="margin-top:8px; margin-bottom:0px; margin-left:6px; margin-right:6px">
<md-input-container class="md-block has-label" style="padding:0px; margin:0px">
<label style="padding:0px">Notification Title</label>
<input type="text">
</md-input-container>
</md-card>
Now it looks quite the same as the Input-nodes: