When an email for one of your contacts arrived in a teammate’s connected mailbox that they had shared with the team, you were alerted about it and could see it on the contact record — but it was missing from your Inbox, and searching your Inbox did not find it either.
Your Inbox now includes that mail, as long as the contact is assigned to you. Each such thread is labelled via <mailbox owner> so it is clear the message lives in a colleague’s mailbox rather than your own, and it opens read-only: replies still go out from your own connected mailbox, so reply from the contact’s email composer (or ask the mailbox owner to reply) rather than from the Inbox thread.
Nothing else changed about shared mailboxes. Sharing a mailbox still does not put a colleague’s mail in everyone’s Inbox — only mail for contacts assigned to you appears.