Hi,
We are not sure if this is a Yoast specific issue. We say this as Yoast is not expected to impact the rendering of an h1 tag on the page.
To help use better understand the issue we ask that you please do the following:
1. Briefly deactivate the Yoast plugin. Doing so will not hurt the SEO of the page.
2. Clear all your caching. This guide explains more: https://yoast.com/help/how-to-clear-your-wordpress-cache/
3. Search the source code of the page for the h1 tag. This guide explains more: https://yoast.com/help/view-page-source-site/.
Does the h1 still fail to render even if Yoast is inactive?