When you are in the Blossom plan (or above), you have the ability to create more than one support email address and map them to the portal for your customers to write to them. These would come in as tickets as expected considering that the forwarding is set up right for these addresses.
When the customer writes an email to the newly configured address, the response from the agent goes from this new address as expected. But when an email is raised on the portal, which is by clicking on "submit a new ticket" - the reply by default goes from the global support email address. Please make sure, this is checked when you are replying to a ticket and intend to send from a different email address. This can be changed by choosing the required address from the drop-down next to the "from" address.