A neutral interviewer is very common. Some interviewers intentionally avoid signaling whether an answer is good or bad because they want to evaluate the candidate independently.

Focus on objective indicators: did you understand the prompt, build a clear structure, handle the math, draw insights, and respond to guidance? Those are more useful than facial expressions or verbal encouragement.

Do not change your style just to get a reaction. Stay concise and keep driving the case. If useful, this guide on first-round consulting interviews gives more detail.