subnet router no access to uplink DNS

[ThomasHa]

Hello everyone,
i'm running opnsense 24.7 on a protectli 4 port appliance. What i want is a subnet-router (i.e. not edge) for a subnet with an uplink to my main intranet as well as internet.
ISSUE: clients from the subnet as well as opnsense itself can't reach the main intranet's DNS/DHCP server.
The reason seems to be an automatically generated route destination DNS/DHCP server which points to the main intranet's gateway. When i delete this route, the DHCP/DNS can be reached.
Whenever i save the opnsense ISC DHCP settings this route is created again.
QUESTION: which feature creates this route and what's the point of it?
SETUP:
 - main intranet: 10.250.0.1: DHCP and DNS router, 10.250.0.254: gateway to internet
 - opnsense: IGC0:interface "UPLINK"/IPv4 configuration from DHCP (10.250.0.1)
             IGC1:interface: "SUBNET"/IPv4 configuration static 10.250.3.254
             ISC DHCP service on interface SUBNET: DNS server: 10.250.0.1
             System|Routes|Status: automatically create route destination 10.250.0.1 gateway 10.250.0.254 flags UGHS

thanks,
Thomas