Messaging API 429 throttling on /api/v2/conversations/messages during JMeter load

No idea why this is happening, the /api/v2/conversations/messages endpoint returns 429 Too Many Requests when JMeter 5.4.1 sends just 10 concurrent POST requests per second. The load test simulates simple text messages for a Digital queue, and the response headers show the rate limit window is exhausted almost instantly.

The environment is Genesys Cloud standard edition with default messaging settings. I checked the Architect flow and it just routes to an agent without complex logic. The error happens before any agent interaction, so it seems like a pure API capacity issue.

Is there a specific rate limit for message ingestion that is lower than the documentation suggests? I need to know the exact threshold to tune the JMeter thread group properly.