AFAIK, the reply button behaves as follows:
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If a staff closes the topic, the button will stay there until the user refreshes the page. If one doesn’t refresh, they will be able to “open” the reply interface (meaning that they will appear as they are writing something down) but once they press “Reply”, a message will appear saying that the topic has been closed.
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I haven’t seen a reply button after a closure, can you perhaps tell them to share a video of that happening? You should try to reproduce it here and post in the bug category.
What I have also seen thousands of times (also happens in your screenshot) is that a mod/admin closes the topic, while a user is still replying. Having said that, their avatar will still appear at the bottom.