TCP connection is rarely closed without returning HTTP response when keepalive enabled in httpd

Issue

• Under high load conditions, httpd rarely closes immediately the TCP connection without returning an HTTP response, only when KeepAlive On is enabled with event MPM. There is no error message in error_log. What are the possible causes?

17829 2022-11-10 11:39:31.759668549    127.0.0.1 → 127.0.0.1    TCP 74 48486 → 80 [SYN] Seq=0 Win=43690 Len=0 MSS=65495 SACK_PERM=1 TSval=2473794689 TSecr=0 WS=512
17832 2022-11-10 11:39:31.759688278    127.0.0.1 → 127.0.0.1    TCP 74 80 → 48486 [SYN, ACK] Seq=0 Ack=1 Win=43690 Len=0 MSS=65495 SACK_PERM=1 TSval=2473794689 TSecr=2473794689 WS=512
17835 2022-11-10 11:39:31.759699871    127.0.0.1 → 127.0.0.1    TCP 66 48486 → 80 [ACK] Seq=1 Ack=1 Win=44032 Len=0 TSval=2473794689 TSecr=2473794689
...
48037 2022-11-10 11:39:32.213938240    127.0.0.1 → 127.0.0.1    HTTP 275 GET /testapp/echo HTTP/1.1  (application/json)
48078 2022-11-10 11:39:32.215448663    127.0.0.1 → 127.0.0.1    TCP 66 80 → 48486 [FIN, ACK] Seq=23220 Ack=10033 Win=95232 Len=0 TSval=2473795145 TSecr=2473795143  <<=== sent FIN, ACK flags from httpd without HTTP response
48092 2022-11-10 11:39:32.215622962    127.0.0.1 → 127.0.0.1    TCP 66 48486 → 80 [FIN, ACK] Seq=10033 Ack=23221 Win=94208 Len=0 TSval=2473795145 TSecr=2473795145
48094 2022-11-10 11:39:32.215634928    127.0.0.1 → 127.0.0.1    TCP 66 80 → 48486 [ACK] Seq=23221 Ack=10034 Win=95232 Len=0 TSval=2473795145 TSecr=2473795145