Thanks for confirming that.
If you’ve used the API appropriately it won’t make a difference. If a topic has a topic_embed record and the site setting Embed set canonical URL is true, then the canonical url will be the topic embed url.
This is a custom open graph tag that was invented by Discourse to improve oneboxing between different Discourse instances. It’s not going to impact the canonical url vis-a-vis SEO because it won’t mean anything to any other platform besides Discourse.
In your screenshot of the HTML I see that the canonical URL is indeed being set correctly. Perhaps the tool you’re using needs to be configured or used differently?