Multiple assignments and none are marked as primary

Summary:

Somehow, an employee has two assignments and after an update, neither one is marked as primary. How do you detect that, what does it break, and how do you fix it?

Content (please ensure you mask any confidential information):

An employee has two assignments. Asg1 is marked as primary. Then, Asg2 has a change and should become the primary. Somehow, Asg1 now does not have the primary flag ticked, but neither does Asg2. I don't know what chain of events led to this, but the UI clearly is letting it slip through somehow.

We spotted this when the payroll team noticed that the Payroll Activity report was erroring. A DBI in a fast formula called by the report shows a local variable being used without being initialized. The logs don't report who it was though, we had to do an xlookup in Excel against another report to find out who was missing from the Payroll Activity Report to track them down. There must be a better way.