Resize your browser window to be slightly less than 850px wide. The “+ New Topic” button will collapse to a + button.
Using a screen reader like macOS VoiceOver, select the button. (I’ve also reproduced this in JAWS on Windows with Firefox.)
Expected: The collapsed button should have an ARIA description, explaining that this is a New Topic button
Actual: The collapsed button has no description. The screen reader just says, “button.”