Yes! It turns out it’s a known issue with Learndash. That link goes to a support log-in for their FAQ, which provides a workaround. To be honest, I only found out about this after stopping using LearnDash, because the problem made the site essentially unusable. So I haven’t tested it for myself.
Coming back to answer my own post, in case it helps someone else down the line: the shortcode (for the individual forum ONLY) was distorted in Visual mode. Switching to Text mode and correcting the shortcode seems to have fixed the issue.