Yes, it seems their servers are terminating responses early. Unfortunately it's undocumented, was introduced unexpectedly one day, and prevents full use of larger context-window models.
It also charges in full for the entire response, which seems to continue running in the background. Definitely a bug at their end.
Since it's 5 minutes I'm wondering if it's just "that's how the web works by default". I remember a while ago I was doing a long-running calculation on an POST request and was trying to debug why the connection always broke after 5 minutes, and found the hard way that there's a 5 minute timeout.
I think it's just some networking misconfiguration they haven't noticed. I can't see them intentionally trying to prevent DoS this way, that's what API key rate limits are for (it would be a very expensive and slow DoS otherwise!).
It also charges in full for the entire response, which seems to continue running in the background. Definitely a bug at their end.