Suitecrm behind nginx can reract with sending email result

Hi guy
just come from my anonther post, that is issue have been solved, but now i have a new one

my suitecrm deploy on k8s cluster, and k8s behind a nginx
the delay between nginx and k8s cluster is more the 250ms

k8s cluster provide a private ip address access point to the suitecrm
nginx provide a public ip address access poiont to suitecrm, reverse proxy mode

when i visit private ip address (less than 3ms), send email with a large size of attachment(13MB), it was succuessfull and suitecrm web UI react with a correct result , “mail sent”
when i visit nignx , send email with a large size of attachment(13MB), i was successfull send but web UI did not react with correct result , just a circling icon.

not sure what cause this issue , delay ? or config.php need to configure a timeout arg ?