We have a corporate proxy and trying to use a custom update-center.json with "archives.jenkins-ci.org" (because it is whitelisted) instead of "updates.jenkins.io/download"
We've noticed that even we change the default URL, the plugins are still trying to be downloaded from updates.jenkins.io following the mirror - see below

Caused: java.io.IOException: Failed to load https://updates.jenkins.io/download/plugins/ldap/701.vf8619de9160a_/ldap.hpi to /var/lib/jenkins/plugins/ldap.jpi.tmp
	at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:1332)
Caused: java.io.IOException: Failed to download from https://updates.jenkins.io/download/plugins/ldap/701.vf8619de9160a_/ldap.hpi (redirected to: https://ftp-nyc.osuosl.org/pub/jenkins/plugins/ldap/701.vf8619de9160a_/ldap.hpi) 

We've replaced all occurences into update-center.json like "updates.jenkins.io/download" with "archives.jenkins-ci.org" and the issue still persists.
Then trying to get the content of the file in different ways:

1. after we disabled authentication (for test), added the file in userContent and changed URL to http://localhost:8080/userContent/update-center.json
2. using file path from system
file:///opt/jenkins/home/userContent/update-center.json
But no luck.

Any ideas?

Jenkins: 2.414.2

OS: Linux - 3.10.0-1160.99.1.el7.x86_64

Java: 11.0.20 - Red Hat, Inc. (OpenJDK 64-Bit Server VM)