I'm having the same issue, however the workaround i have come up with as the emails get sent via a 365 mailbox (if set up like that) you can set up a BCC rule in the exchange admin.
Go to... Exchange Admin Center > Mail Flow
*Apply this rule if... "The sender is..." (add in the mailbox sending the emails)
*Do the following... "Bcc the message to..." (add in users to Bcc in)
As the Invoices are being sent via a specific user it copies the emails from that sent mailbox to the required email.