I just encountered this same issue (a spider crawl indicated that all H1's were missing) when in the process of optimizing a client's SEO on their Squarespace 7.0 site.
As you can see from what @Boise_Jim shared, there are no H1's showing up in the crawl report, and I'm having that same result. I'm not sure why Squarespace would wrap the site-title or logo in an <h1> tag, when there is likely to be a heading on the page, where a user can apply "heading" (H1) accurately.
I'm also not following the logic @edharris. If the site-title/logo has an <h1> tag, and then the actual heading has an <h1> tag, and Screaming Frog is scanning for the first 2 instances of <h1> tags, then the heading should appear in the report, but it is not.
For On-page SEO best practices, there should only be one <h1> tag on any page. While Google is smart, it does create indexing issues when tags are not applied appropriately (or in this case when there are multiple <h1> applied when Google is expecting only one), which can negatively affect SEO.
Are we able to remove the <h1> tag from the site-title/logo and instead use a better tag for this, such as:
figure > img - for image logo
div > span - for text logo
FWIW, when I viewed the page source for multiple pages on my client's Squarespace website, the only <h1>s that appeared in the code were applied to logo. No <h1> tag was applied to the actual heading in the code, even though "Heading 1 / <h1>" was applied in the text styling.
