JMeter 503 errors on /api/v2/architect/flows/publish at 10 threads

Context: Running JMeter 5.6.2 from Asia/Singapore. Targeting /api/v2/architect/flows/publish with just 10 concurrent threads. Getting 503 Service Unavailable immediately. No rate limiting headers present.

Question: I cannot figure out why the platform rejects valid publish requests at such low concurrency. Is there a hidden throttle on flow updates?