I looked up the last screenshot I had of this. What I asked was "In 2 days remind me to call FRIEND_NAME", and Siri created a reminder that just said "call". Transcribed perfectly, wrong content in the reminder.

I tried it again right after, and the reminder said "call FRIEND_NAME".

I don't think there was any previous conversational context or anything like that. Hard to fathom how that could happen.