You appear to be able to change the setting as per the NSX documentation, but the setting does not persist.
VMware NSX Data Center for vSphere
vCloud Director
Edges that were deployed and managed through vCloud Director, should not be modified using NSX Manager. Rather, they should be adjusted or configured through vCloud Director.
You see the successful edit in NSX, is because it does succeed, but only temporarily. As vCloud Director synchronizes its settings, the Edge is changed back so that the setting is whatever it was configured to be in vCloud Director.
Note: Current releases of vCloud Director allow setting of the Syslog server for Edges at a Global level, not a tenant and vCloud Director synchronizes its settings with the Edges on a regular basis.