The website is not showing canonicals the right way. If you look at hidden link and hidden link (same page, different language), the canonical is correct.
But if you look at hidden link and hidden link, the English version has a canonical to the Dutch website.
I've tested the following:
- Deactivate all plugins
- Use a default template: Twenty Twenty (I've added cases as Custom Post Type)
- Activate Yoast SEO Plugin (which sets canonicals)
- Activate WPML Plugin (which sets languages)
With above settings, the problem still persists. If you'd like to test this, i can send you a link to a test website.
Canonical Tags can either be self-referencing (where a canonical tag point to a page's own URL) or can reference another page's URL to consolidate signals.
Documentation: hidden link
With Yoast you should be able to set your canonical within the page, if possible edit your page and review your settings for the canonical within the page
I was not aware that it was a post archive page, please review the following errata and keep up to date with it as once the issue has been fixed it will be updated
