Forwarded emails from agents' email clients (e.g. drag-and-drop from Thunderbird to the osTicket mailbox) are bypassing filters and going to the default department and help topic since upgrading.

Check the raw email headers of a forwarded message -- when an email is dragged/forwarded, headers change entirely (new From, new Message-ID), which may not match your filter rules anymore. Review your Email Filters in Admin Panel > Emails > Filters and update the matching criteria to account for forwarded message header patterns. Post the raw headers and filter config in a new thread for more specific debugging help.