their crosspost was missing the additional information they added in the body of the original post

What do you mean "missing"? Are cross posts supposed to copy over the original post body? Post bodies often contain notes specifically for the community they are posted in and you wouldn't want the original body. Just the link and your own post body. Even if they are copied, you can certainly remove that text and write your own. Hell, you could just delete the "crossposted from" note, too!

This is not a bug. It's a tiny feature the web UI does that Thunder chose not to implement.

(I fixed your nouns in the quote. You should have just done that in your edit.)

Edit: indeed, the web UI quotes the entire original post body and adds the cross post note. You are free to delete all of it.

I realize, in this case, crossposting this post without also adding a body explaining the post is a little odd. But it's always the poster's choice to add or modify the post body after clicking crosspost. Could Thunder add a feature to copy the original body as a quote? Sure. But it's not a big.