Branch calls turn choppy every evening although QoS shows no drops

A branch router R1 connects to the MPLS provider over a 1 Gbps fibre port, but the contract is 50 Mbps. A QoS policy WAN-EDGE (voice in a priority queue, critical data with a bandwidth guarantee, fair queuing for the rest) is applied outbound on the WAN port. Every evening from 18:00, when the nightly file sync to the data centre starts, staff calling head office complain of robotic, choppy audio. Calls inside the branch are fine. The previous engineer says "QoS is configured and shows zero drops, so it must be the provider".