It's definitely possible because I've had issues in the past where users don't bother to populate the planned delivery and GR processing times in the contract and MRP uses zero instead of the values maintained in the material master.
There is an indicator in configuration at plant level that you could try, it's description suggests it only affects planned delivery time but I have a feeling it affects both values. It's under the External Procurement button in the "Carry Out Overall Maintenance of Plant Parameters".