Hi Vikash
I think this behaviour is coming up due to the usage of the Enhanced UWL People Picker. You can check further details at the following page:
Enhancements to the Universal Worklist People Picker - Business Process Expert - SCN Wiki
Also the following HELP page talks about it:
Configuring the People Picker - Configuring the Universal Worklist - SAP Library
As the second link explains, to grant users the rights to search in all companies (in case you don't implement the companies concept in your scenario) you have to assign the UWLSearchUnrestricted UME action to the respective user that needs to perform search.
Check this and let me know if you are able to do it!
Cheers and regards,
Armando Zaro