Azure Virtual Machine Scale Set error: The selected virtual network has a default subnet with a reserved name

What makes a subnet as default if not the name or lower starting IP? Getting below error on selecting a virtual network:

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Below are the subnets configured
Name                IPv4            Available IPs
default             10.0.0.0/24     251
GatewaySubnet       10.0.1.0/24     availability dependent on dynamic use
AzureFirewallSubnet 10.0.3.0/26     56
Bot-subnet          10.0.4.0/24     251