Email threading replies not showing up

This question has been asked before (https://discussions.apple.com/thread/250705248), but no useful answer was given.

If you look at the thread in the image below, you can infer that Laura must have received mails from me in between most of her mails. However, they are not visible.

They do exist in the sent mail folder, i.e. in the folder designated as the sent mails folder. When I send mails, they end up in the folder marked yellow.

Here are my mail threading settings:

Best regards,

Hans

[Image Edited by Moderator to Remove Personal Information]