Messaging API 429s during high-concurrency JMeter tests

So I’m seeing a very odd bug with the Messaging API. When JMeter threads exceed 100 concurrent requests to /api/v2/conversations/messages, the US1 environment returns 429 Too Many Requests. This happens even with valid OAuth tokens. The error rate spikes significantly after 5 seconds of steady load. Is there a specific rate limit for digital messaging endpoints that differs from voice APIs?

Thanks for any insights.