This issue is introduced during the upgrade of NSX-T 3.1.x release to NSX-T 3.1.x release, where the proxy setting page splits into two (CEIP, Proxy Setting).
During the upgrade, the UI triggers an API PUT that removes Proxy configuration.
Also, if proxy setting is disabled, any modification of telemetry settings (CEIP) from the UI overrides the proxy setting and resets it to null. This results in loss of proxy configuration.