I received answer from the team, looks like for new accounts (like mine, 1 month old), there is a restriction for CC and probably BCC. The limitation is now removed and it works as expected.
A suggestion to InvoiceNinja team - put some information on this (probably even disable the fields with proper explanation banner), I’ve lost several hours of testing and debugging on this simple feature, just to understand it is disabled on the InvoiceNinja side.