Quick question about OAuth token refresh 401s under load

Quick question about security token handling in US1. running jmeter 5.6 to simulate 300 concurrent api requests using client credentials grant. the goal is to test throughput near rate limits.

getting 401 unauthorized errors after exactly 359 seconds, even though the refresh token logic is in place. is there a hidden concurrency limit on the /oauth2/token endpoint or a specific cache behavior i am missing?