Why is the archive box icon button not closing in the inbox?
In a team inbox the box icon always closes the conversation.
In your Inbox:
If you are assigned to the conversation, the box icon will prompt you if you want to archive & close, or just archive.
If you are not assigned to the conversation, the box icon archives the conversation.
Say user A is assigned to a conversation and mentions user B to ask a question. User B will see the conversation in their inbox, reply to the question, and they will want to archive the conversation from their inbox, but they may not want to close it because this conversation is still assigned to user A.