Image two spam messages are posted at the same time.
You open one and flag it as spam and it disappears.
You open the next and the spam radio button in the pop-up will be selected but, I suppose the model (model in MVC) won’t as the state of the pop-up won’t reflect that. You have to click the already selected radio button to tell Discourse that this post is spam.
I believe this is a bug and am marking it as such.