diff --git a/agent/configentry/merge_service_config.go b/agent/configentry/merge_service_config.go
index f11d96fcc5..b131b7e7a7 100644
--- a/agent/configentry/merge_service_config.go
+++ b/agent/configentry/merge_service_config.go
@@ -173,11 +173,21 @@ func MergeServiceConfig(defaults *structs.ServiceConfigResponse, service *struct
 			us.MeshGateway.Mode = remoteCfg.MeshGateway.Mode
 		}
 
+		preMergeProtocol, found := us.Config["protocol"]
 		// Merge in everything else that is read from the map
 		if err := mergo.Merge(&us.Config, remoteCfg.Config); err != nil {
 			return nil, err
 		}
 
+		// Reset the protocol to its pre-merged version for peering upstreams.
+		if us.DestinationPeer != "" {
+			if found {
+				us.Config["protocol"] = preMergeProtocol
+			} else {
+				delete(us.Config, "protocol")
+			}
+		}
+
 		// Delete the mesh gateway key from opaque config since this is the value that was resolved from
 		// the servers and NOT the final merged value for this upstream.
 		// Note that we use the "mesh_gateway" key and not other variants like "MeshGateway" because