Domain resolution problem between GCP CloudDNS and Goolge Domains

Question

I've 2 applications:

1. Backend running on GKE cluster
2. A static website (landing page) on Google Firebase.

Both apps use wildcard domain (example.com for static website and backend.example.com for the backend) bought from Google Domains. Firebase (static site) app's DNS records are present in Google Domain's default DNS servers where as Backend's DNS records are in GCP cloudDNS. Backend is exposed via GCE ingress controller over HTTPS using GCP's Managed Certificate service.

The problem I am facing is that I am only able to access the Firebase application, not the other one. nslookup and dig both are able to resolve example.com not backend.example.com.

~ ﴱ (gke_cluster-1) > nslookup example.com
Server:     127.0.0.53
Address:    127.0.0.53#53

Non-authoritative answer:
Name:   example.com
Address: 34.111.182.171

~ ﴱ (gke_cluster-1) > nslookup www.example.com
Server:     127.0.0.53
Address:    127.0.0.53#53

Non-authoritative answer:
Name:   www.example.com
Address: 199.32.158.100

~ ﴱ (gke_cluster-1) >nslookup backend.example.com
Server:     127.0.0.53
Address:    127.0.0.53#53

** server can't find backend.example.com: NXDOMAIN

~ ﴱ (gke_cluster-1) dig +short example.com
34.111.182.171
~ ﴱ (gke_cluster-1) dig +short www.nozl.devwww.example.com
199.32.158.100
~ ﴱ (gke_cluster-1) dig +short backend.example.com
# no output

If I remove the firebase app and it's associated DNS records from Google Domain, then backend.example.com works fine.

Does someone know how to troubleshoot this?