New ticket is created in a different department when somebody add a reply to ticket

New ticket is created in a different department when somebody add a reply to ticket

Hello,

We have a little problem when somebody reply to a ticket from email( not sure if it is happening also when reply is from customer portal) a new ticket is created in our default department with the same ticket number as original ticket and in Subject is used "RE"+ $ticket_number + $original_ticket_subject.
The new ticket created is not assigned to anyone.
This is very confusing for us because we miss our customer's replies to tickets and also creates too many tickets.

Please let me know if you or I can solve it.

Thank you,
Andreea Mihaela Petrea