Allow display settings to override stream wrappers that implement getLocalThumbnailPath()

Hi,
I have a content type with a field "video" (fieldtype: file, widget: mediabrowser).
Then I've selected "Magnific popup":
Gallery Type: all items displayed
Image Thumbnail Field: video
Thumbnail Style: "video style"

On the content type page (front-end) I see this:
<a href="[video-url-youtube]" class="mfp-item" style=""><img class="mfp-thumbnail" typeof="foaf:Image" src="[site-url]/sites/default/files/media-youtube/W58Zm6G_X5Q.jpg" alt=""></a>

Why is it using "media-youtube"?
Why is it not using the image style I have selected?
I have an other site where I am using the same, and there it is working fine ... I don't get it ...