Note that this does not impact iOS VoiceOver, because on iOS, the New Topic button doesn’t collapse; the navigation collapses into a drop-down menu instead.
We’ve been mapping the title attribute to aria-label and it looks like macOS’s VoiceOver ignores it.
What I’ve done is to map the label attribute (which is the visible text) to the aria-label attribute. A quick search shows up that 268 d-buttons have labels and only 69 have a titles, so other parts of the interface will also be improved.