H1 Tag renders as H2 tag in modern page text webpart

Jaymz Yates smblair72 Darren70s Daniel TSHIN 

This is known behavior in SharePoint online. As per Microsoft:

Many Web Parts introduce a level 2 heading to pages without providing a means for users to change that heading level.

For headings added to pages using the Text Web Part, the heading level the page author specifies is downgraded by one when presented to the end user (e.g., a heading a page author sets to level 1 will be presented as a heading level 2 in the final HTML). The purpose of this is to ensure that the page title is always the only heading level 1 on the page and to avoid conflicts with heading levels when users copy and paste from other sources.

Tables added with the Text Web Part do not have column or row headers defined.

Find more details at: Accessibility Question about Heading Levels and Heading Styles 

I will suggest you to:

1. Raise a support ticket with Microsoft directly and report this: Get M365 support - online support 
2. Submit feedback/idea on SharePoint Feedback Portal 

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