Just in case you are interested I made a post on my client apps community about it. It seems the webUI does things different than Thunder in terms of crossposts. Not sure about other clients, we will see if the Thunder devs decide to change that or not.
That's how it always looks? Do you have a screenshot of something that says otherwise? Just curious because I thought it is not possible on Lemmy to see which one is the original post. Also have no idea how a specific client could not adhere to how to do that.
Edit: sorry, just noticed now that the second screenshot shows what I asked.