Quick question about PR API 503s at 50 threads

Quick question about Predictive Routing outbound campaign updates failing under load.

JMeter 5.6.2 from Singapore hitting POST /api/v2/predictiverouting/outboundcampaigns. Single thread works fine. At 50 concurrent threads, getting 503 Service Unavailable on the initial request, not just rate limiting.

jmeter_config:
 thread_count: 50
 ramp_up: 10
 loop_count: 1
 target_endpoint: /api/v2/predictiverouting/outboundcampaigns
 region: us-east-1

Is there a hard limit on concurrent campaign creation requests per org? Or is this a backend timeout issue?