Hi @luke2,
My guess is that most free SEO heading tag checkers don’t [yet] dynamically render JS, so they simply don’t “see” the Bubble heading tags. This tool, however, does see the H1 tag on my Bubble page.
There is in fact just a single H1 tag on the page. I’d be curious to know if it works for others as well. The page I tested is the same one I checked directly using Google’s search console, as noted here. (It’s dynamically generated using Sudsy Page.)
Google also has a relatively new tool, and these are the results it yields for the same page…
Again, the page I tested is not on a custom domain, and zero effort was put into optimizing.