Could you provide a link to the page so we can take a look?

This issue is because Fandom currently required TOCs to be at least 400px wide. On smaller screens/windows, there is not enough room for both the infobox and the TOC so the TOC gets pushed below the infobox. If you would really prefer to have the TOC squished rather than pushed, you can add the following to MediaWiki:Fandomdesktop.css.

.toctitle {
    min-width: auto !important;
}

I usually discourage people from using !important in their CSS but it truly is required this time because the min-width is set in the TOC's style attribute.

Thank you! This is exactly what I was looking for, I would have never figured this out on my own.