Comment 2 for bug 1790038

The "BadRequestException: Unknown error" error is a generic one as neutron-server rejects the request (proper error is displayed in Rocky, you may have more info with debug enabled).

The actual error:
$ openstack subnet create --subnet-pool pool2 subnet2 --network testnet
BadRequestException: 400: Client Error for url: http://10.41.1.22:9696/v2.0/subnets, {"NeutronError": {"message": "Subnets hosted on the same network must be allocated from the same subnet pool.", "type": "NetworkSubnetPoolAffinityError", "detail": ""}}

If the pools you used are the ones listed in comment 0, you most likely have the same result.

I cannot find a relevant report/review on the CLI not reporting errors properly, but on the neutron side I would say this is the intended behavior (rejecting that second subnet)