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Reply All from Shared Mailbox removes Sender Address and Duplicates Shared Mailbox Address
Most likely becaus Outlook "sees" you as PersonA. So how exactly are you accessing the shared mailbox? This is the expecte behavior when you have the shared mailbox added as additional mailbox (via File -> Account settings -> More -> Advanced), as you are acting as the delegate. If you want the "correct" behavior, add it as additional account instead (File -> Add account -> enter shared mailbox address -> enter *your* username and password when prompted for credentials).
We're giving PersonA full permissions on that Shared Mailbox, which in Outlook means we don't add the mailbox at all, Outlook does it by itself.
So Automapping, got it. This is equivalent to the "additional mailbox" scenario, thus the behavior you are seeing. If you want the "correct" behavior, you need to add it as additional account instead.
From what you're saying it sounds like Automapping is optional? How do I switch it off and try what you're suggesting?
It's optional, but need to be configured via PowerShell. Remove the permissions and re-add them with Automapping turned off:
Remove-MailboxPermission mailbox -User delegate -AccessRights FullAccess
Add-MailboxPermission mailbox -User delegate -AccessRights FullAccess -Automapping $false
The mailbox will not automatically appear in Outlook then. To add it (as additional account), use the File menu -> Add Account -> Enter the shared mailbox email address and dont enter password. When prompted for credentials, enter the UPN/password of the delegate user.