The connection with the sftp is not consistent.

[Dregos13]

Since we have started using sftpgo we have had several setbacks. The most severe of them is that when connecting the client disconnects randomly dropping the connection and losing the file in question.
To deploy sftpgo we have a kubernetes cluster and in this cluster we configure the ftp with a NLB:

containers:
- image: drakkan/sftpgo:v2.5.0-plugins
envFrom:
- configMapRef:
name: sftpgo-env
imagePullPolicy: Always
name: sftpgo-app
ports:
- name: http
protocol: TCP
containerPort: 80
- name: ftp
protocol: TCP
containerPort: 2021
- name: ftp-passive
protocol: TCP
containerPort: 20
apiVersion: v1
kind: Service
metadata:
labels:
app: sftpgo-app
name: sftpgo-service
namespace: staging
annotations:
service.beta.kubernetes.io/aws-load-balancer-type: nlb
service.beta.kubernetes.io/aws-load-balancer-eip-allocations: eip
service.beta.kubernetes.io/aws-load-balancer-subnets: a/b/c
service.beta.kubernetes.io/aws-load-balancer-nlb-target-type: ip
service.beta.kubernetes.io/aws-load-balancer-scheme: internet-facing
service.beta.kubernetes.io/aws-load-balancer-manage-backend-security-group-rules: "true"
service.beta.kubernetes.io/aws-load-balancer-type: external
service.beta.kubernetes.io/aws-load-balancer-security-groups: sg
spec:
ports:
- name: http
protocol: TCP
port: 80
targetPort: 80
- name: ftp
protocol: TCP
port: 21
targetPort: 2021
- name: ftp-pasivo
protocol: TCP
port: 20
targetPort: 2020

Why does the client disconnects like this?