Hi Hans,
Thanks for your patience. I've found the cause.
The "Show Date" option in the Articles addon shows the article's Start Publishing date, not its Created date. On your site, the Start Publishing date of all articles is set to 29 July 2026, probably from the migration or a bulk update on that day. Many articles were also last modified that day, so both dates look the same. For example, "Ervaringen met Thuisbatterijen" was created on 17 June 2026, but its Start Publishing date is 29 July 2026.
To fix it, set each article's Start Publishing date (Articles > Edit > Publishing tab) to its Created date. With many articles, this can be done in one step with a database query (please take a backup first):
UPDATE #__content SET publish_up = created;
(Replace #__ with your table prefix.)
Let me know the outcome.
Best regards,