It's the director's fault.
A similar problem happened in the UK last year with "Jamaica Inn" - the dialogue was mumbled with a Cornish accent and the BBC got thousands of complaints.
The sound recordist at the time had flagged this up with the director and was overridden.
A sound recordist cannot compensate for poor acting ability.
The problem is that the director has the script and knows what the actor is saying, forgetting that the audience will have no script and has to rely solely on the spoken word.
Definitely a production fault and not in any way down to the sound recordist.
Absolutely 100% in agreement, especially with the script part. I've had this exact thing happen to be when I used to edit video game dialogue. A particular talent was reading through a line really fast, and since I had the script I understood it. Someone happened to walk into my edit room while I was working on a line and they said "what the hell are they saying. It's too fast and I can't understand it". So I ditched the script and just listened to the audio, and sure enough, as clean as the audio was, I still couldn't understand half of it because the talent just spoke way too fast, and it wasn't caught until the edit after the talent was gone. They had to call them in and re-record everything.