Random SSL cer expire after renewing a new one

Hi everyone. I’m using a Kong with version 2.3.3 on Ubuntu. I config it inside the kong.conf like this,

ssl_cert = /etc/ssl/ssl_certificate.crt
ssl_cert_key = /etc/ssl/private.key

The old SSL certificate worked perfectly. But after I replace the new cert to,
ssl_cert = /etc/ssl/2022/ssl_certificate.crt
ssl_cert_key = /etc/ssl/2022/private.key

Sometime the request send to a Kong work perfectly, sometimes it return an error 'certificate has expired’

It feels like sometime Kong returns the old cert to clients.

I really have no idea how to solve this. Please help.