Ahrefs. Which I would say is a pretty reputable SEO tool, no?
Ahrefs is a great tool, but (evidently) it only checks the initial page html for H tags (not the processed DOM)…
UPDATE On The Above: actually the default setting in Ahrefs is not to execute JavaScript when when doing crawl analysis… so it will only check the initial HTML (hence why your H1 tags are not picked up)…
If you want it to execute JavaScript when crawling (which you do) there is a setting to do just that… so just turn it on then do another crawl analysis and your H 1 tags should show up (mine certainly do).