I have four iframes on four separate pages. When published, three of them have a white line at the bottom of the placeholder.

Here's the code I'm using (only the URL changes)

<iframe src="https://auzlink.net.au/main80"
width="100%" height="100%"
frameborder="0" >
<p>Your browser does not support iframes.</p>
</iframe>

I copy/paste into the mobile variant and no line. Copy/paste into a new page, line.

Link to one of the pages > https://auzlink.net.au/80.htm < If anyone is inclined to hit the play buttons on the HS (Hardstyle) page, be warned the lyrics are very explicit and clearly heard.
Xara file > Auzlink TG.web <