Scenario:
A report with custom column formula which includes Billing related Fields disappears automatically.

Cause:
If Use Billing information is disabled under the Administration section of System Preference,  this will cause anything related to billing disappear. Once this option is disabled, reports will no longer have the billing related fields (Eg: BillableHours, User Default Billing Rate…etc) and hence any custom column which includes billing related fields in calculation is no longer visible in reports.