I've tried to debug OVMF code and have found out that the RouteConfig
call fails because the DevicePath is not present in the ConfigResp
string:
https://github.com/tianocore/edk2/blob/7c0ad2c33810ead45b7919f8f8d0e282dae52e71/MdeModulePkg/Universal/HiiDatabaseDxe/ConfigRouting.c#L5486

I think this implementation guards against the possibility that you might have two similar devices that install the same third-party driver. Now the two drivers will install two HII forms with the same GUID so the RouteConfig has to distinguish between then somehow.

Cheers,
Tom