"Auto-detect" WAN gateway breaks client outbound connection

[Apollo13]

Hi folks,

I've got a strange problem I'd really like to get support on.
My system has two interfaces:
LAN has IP range: 10.0.0.0/24
WAN has a public static address - let's say it is 192.0.0.100/24. Additionally I configured one (and only one!) gateway with IP 192.0.0.1 and checked the "default" box.

Now pings to a linux system inside the WAN net via "Interfaces->Diagnostics->Ping"  work as expected. But if I try the same on one of my LAN clients no ping succeeds.
That only changes if I manually force the WAN interface to use the gateway, i.e. changing the gateway setting from "auto-detect" to the configured GATEWAY entry. Then I've got no problems at all reaching systems that are not on the LAN.

The help text shown next to the gateway options says:

If this interface is a muti-WAN interface, select an existing gateway from the list or add a new one using the button above. For single WAN interfaces a gateway must be created but set to auto-detect. For a LAN a gateway is not necessary to be set up.

Why do I have to set the gateway explicitly?

Thanks in advance!

EDIT: My system is running OPNsense 18.7.10.