The Discourse fix makes sense, but I’d be careful about treating multiple H1s as an automatic ranking penalty. The bigger concern is whether the page has a clear primary heading and whether the heading structure accurately reflects the content. Google also considers heading elements and the main visual title when understanding pages and generating search result titles.
For sites with lots of topic, category, and tag templates, I’d audit the rendered HTML after the update rather than assuming the template change solved everything. Checking the actual output is useful because theme components or plugins can sometimes introduce another heading unexpectedly.
I usually check heading structure alongside title tags, meta descriptions, canonicals, internal links, and other on-page elements.
The most useful test here would probably be to compare a sample of topic, category, and tag pages after the Discourse update, then watch Search Console over the following weeks. That would give a much clearer picture of whether the template change had a measurable effect rather than attributing a traffic change to H1 count alone.