To fix the issue you need to copy below 3 files from working vCenter (exact build as non-working) from the directory /etc/vmware/vsphere-ui/cm-service-packages/com.vmware.cis.vsphereclient.plugin

[ /etc/vmware/vsphere-ui/cm-service-packages/com.vmware.cis.vsphereclient.plugin ]# ls -ltrh
total 12K
drwxr-xr-x 3 vsphere-ui users 4.0K Feb 10 11:46 com.vmware.vum.client-6.7.0.41422
drwxr-xr-x 3 vsphere-ui users 4.0K Feb 23 15:11 com.vmware.fallback.mixed-6.5.0
drwxr-xr-x 3 vsphere-ui users 4.0K Feb 23 15:11 com.vmware.vcIntegrity.vcIntegrity-6.7.0.41422

Then run below command:

chown vsphere-ui:users * 

Restart vCenter services and you should be able to see the update manager plugin.