By Tickets 1 you mean Invoices 1?
I've encountered the same issue. Only way around it I've seen is to add a field to Invoices that either auto-enters or is a calculation referring to the related sort field. (Auto-entered sorts much quicker, but takes scripted support to update should the original field in the related record be modified.)
Yes I did mean Invoices 1, sorry. Anyway, I went with the calculated field since the portal in question displays a small set of records so the difference in sorting is negligable.