Hey Paula thanks for the flag on this! I've had a look and we don't actually set headers to be bold specifically, they're just styled as such. When you copy/paste elements on Mac (maybe similar on Windows) it copies the styling too, so it'll carry over the heavier font weight.

There might be something we can do to tweak this but there isn't a super obvious fix that comes to mind, but will bear this one in mind and open to ideas.