How do you run discovery interviews when customers only want to talk about bugs?

I'm running discovery for a new scheduling feature, and every interview turns into a support session within five minutes. Customers have a list of bugs and complaints, and they've been waiting for someone to listen. I don't want to shut them down, because the frustration is real and hearing it builds trust. But I end up with 40 minutes of bug reports and 5 minutes about the problem I'm actually trying to understand.

So far I've tried saying up front that we'll cover their issues at the end. That mostly works with managers and not at all with frontline users. How do you handle this?