Had a quick look because I was confused.

All posts show as "crossposted to:" in the small space between the Fediverse, star etc icons and the post

What did not happen in your case is having "crossposted from" in the body of the text. Probably a client issue indeed (or maybe a decision, as IIRC Thunder aggregates comments from all crossposts together? Or maybe it's another client?)

I have noticed that posts leading to the same URL all have that little "Cross-posted to:" indicator. Whether the post had actually been crossposted to the indicated community or if it had instead been crossposted from that community. Even if the post was not crossposted at all at any point, it will still read "Cross-posted to:" and list every other post with that URL.

Just wanted to chime in here! I think the implementation of cross-posts is up for interpretation (and can definitely be changed if one is preferred over the other). Generally speaking, there are two types of implementations:

Cross-posts only copy over the title and link of the original post

• This is what Thunder currently does, unless it's a text-only post (in which case, it copies over the text content of the original post since that's the only relevant part of the post)
• The idea here is that we typically want to change the contents of the cross-post to be relevant to the community that we post to.

Cross-posts copy all content of the original post (title, link, body)

• This is what Lemmy UI does, but it injects "crossposted from" into the body of the post
• This can be useful if you want to post the same post across multiple-communities with the same body

From this discussion, I do think that it could be valuable to add an easier way to inject the original body of a post into a cross-post. If you would like this to be a feature, please create a new feature request on GitHub!

A side note on cross-posts:

I believe that Lemmy considers any posts with the same link as a cross-post. There's no distinction (from the API) on which post was the "original" post, or whether a post was crossposted "from a community" vs "to a community". This is also the reason why there is additional metadata for each "cross-post" in Thunder. The additional metadata allows you to figure out when a given post was created to find the "original" post.

If you're curious about specific implementation detail, @micahmo@programming.dev might be able to answer them!