One day shows a negative margin. Is that a bug?

Usually not. A quiet day still carries its share of fixed costs, and a day with several refunds from a busy earlier week can genuinely give back more than it earned. Both are true statements about the day rather than errors.

What to check is whether the same day is negative at CM2, before advertising and overhead. If it is, the orders themselves lost money, which is a pricing or cost problem rather than a calendar one.