Data Action 504 Timeout with JMeter 100 Threads

Could someone clarify why the Data Action endpoint returns a 504 Gateway Timeout when JMeter hits 100 concurrent threads on US1? The flow is just a simple REST call to an external API with a 5s timeout. JMeter 5.6.2 is configured with keep-alive and the error happens consistently after 30 seconds of ramp-up.