For your information, here is some tickets from yesterday that is also zero, and I have personally watched and monitored what was going on on the floor the whole time:
As you can see, viewing the ticket list, some tickets have Zero Total
When you open the ticket details it is blank but indicates PAID:
I have a new theory that may lead to a possible answer:
Is it possible that tickets are blanked if the tickets are moved to new tables?
As in this case the tickets for some of those tables were for a tour group (bus with lot’s of foreign visitors) and it was moved to one table in order for the tour guide to pay only one bill.
Hope this makes sense?