What is the value of your Discourse (Discourse, not WP Discourse) site setting embed unlisted?
Thanks! It was checked per above screenshot.
@angus Is there a changelog/timestamp of admin settings?
You can see that in your logs (Admin Panel > Logs > Staff Actions > Change Site Setting), however it was probably checked for some time. There was an update to the behaviour of that setting in core Discourse, to apply it consistently across embed scenarios, which includes WP Discourse topics. Despite the name and description of the setting, it previously only applied to a subset of embed scenarios. See further