Question:

Why does a Validation Rule work in CRM Online but not Offline for formula fields?

Answer:

Formula fields are evaluated at sync time and are subject to the pre-existing limitations of the Offline application. If formula fields are included in the Validation Rule expression, the evaluation of the formula itself and ultimately the validation rule is subject to the current formula field behavior. The same applies to roll-up summary fields.

Related Documentation:

CRM Help Documentation: Offline Validation Rules