Hi! Welcome to the Atlassian Community!
Could you provide some screens or explain a little more? I have trouble understanding what you are doing and also what you are trying to achieve.
What is "label(BPO) option"?
Are you speaking of a company or a team managed project?
Are you using an automation of some kind that is supposed to set the assignee via label?
You probably checked already, but maybe this has to do with default assignee in the projects settings (project lead, component lead...).
Also it is not possible to assign a ticket to a team as only one person at a time can be the assignee/owner. However there are solutions to work around that (see Solved: Can I assign more than one person to an issue on a... (atlassian.com)).
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