I noticed thumbnails and meta description aren't working on discuss.tchncs.de sometimes when sharing a link. When I view the same post on feddit.org, the thumbnail and description is visible.

For example:

https://discuss.tchncs.de/post/32817362
no thumbnail, no description

https://feddit.org/post/9581626
thumbnail & description visible

I haven't detected a pattern yet. It just seem to work sometimes and other times it doesn't.