HorizontalPodAutoscaler scales up pods but then terminates them instantly - kubernetes

Question

So I have a HorizontalPodAutoscaler set up for my backend (an fpm-server and an Nginx server for a Laravel application).
The problem is that when the HPA is under load, it scales up the pods but it terminates them instantly, not even letting them get into the Running state.
The metrics are good, the scale-up behavior is as expected the only problem is that the pods get terminated right after scaling.
What could be the problem?
Edit: The same HPA is used on the frontend and it's working as expected, the problem seems to be only on the backend.
Edit 2: I have Cluster Autoscaler enabled, it does it's job, nodes get added when they are needed and then cleaned up so it's not an issue about available resources.
deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: fpm-server
labels:
tier: backend
layer: fpm
spec:
replicas: {{ .Values.replicaCount }}
selector:
matchLabels:
tier: backend
layer: fpm
template:
metadata:
labels:
tier: backend
layer: fpm
spec:
{{- with .Values.imagePullSecrets }}
imagePullSecrets:
{{- toYaml . | nindent 8 }}
{{- end }}
containers:
- name: fpm
image: "{{ .Values.fpm.image.repository }}:{{ .Values.fpm.image.tag }}"
ports:
- name: http
containerPort: 9000
protocol: TCP
env:
{{- range $name, $value := .Values.env }}
- name: {{ $name }}
value: "{{ $value }}"
{{- end }}
envFrom:
- secretRef:
name: backend-secrets
resources:
{{- toYaml .Values.resources | nindent 12 }}
hpa.yaml
apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
name: fpm-server-hpa
labels:
tier: backend
layer: fpm
spec:
scaleTargetRef:
apiVersion: apps/v1
kind: Deployment
name: fpm-server
minReplicas: {{ .Values.autoscaling.minReplicas }}
maxReplicas: {{ .Values.autoscaling.maxReplicas }}
metrics:
{{- if .Values.autoscaling.targetCPUUtilizationPercentage }}
- type: Resource
resource:
name: cpu
targetAverageUtilization: {{ .Values.autoscaling.targetCPUUtilizationPercentage }}
{{- end }}
{{- if .Values.autoscaling.targetMemoryUtilizationPercentage }}
- type: Resource
resource:
name: memory
targetAverageUtilization: {{ .Values.autoscaling.targetMemoryUtilizationPercentage }}
{{- end }}

Answer 1

Seems that the problem was with the replicas: {{ .Values.replicaCount }} definition. It seems that, if you are using HPA, replicas can't be used. I removed this line and the HPA started scaling.