Docker network not using all NIC's - docker

Question

My docker container is not reachable on all host network interfaces.
My host server has 2 network interfaces (and ip adresses). When running my docker container without a specific defined docker network, it works and the container is reachable on both ip adresses.
But when I'm running with a self defined docker network and add it to the docker-compose file only 1 ip is working. The other timesout. Why does this happen?
Docker-compose file
version: '3.7
services:
servicename-1:
#network_mode: "host"
image: nginxdemos/hello
init: true
ports:
- 8081:80
volumes:
omitted
environment:
ommitted
networks:
- a-netwerk-1
networks:
a-netwerk-1:
external:
name: a-network-1
docker inspect network:
[
{
"Name": "a-network-1",
"Id": "df4ab5e3285c75b71f8f88f66c4c5d85ad8f2f9b17e66f960b11778007810b96",
"Created": "2020-01-30T10:55:14.853289976+01:00",
"Scope": "local",
"Driver": "bridge",
"EnableIPv6": false,
"IPAM": {
"Driver": "default",
"Options": {},
"Config": [
{
"Subnet": "172.29.0.0/16",
"Gateway": "172.29.0.1"
}
]
},
"Internal": false,
"Attachable": false,
"Ingress": false,
"ConfigFrom": {
"Network": ""
},
"ConfigOnly": false,
"Containers": {
"2f2d5b2e22b3066085246ea53d1ca2c9f963b5e9138ae7202d8382be98428476": {
"Name": "test_testservicename_1",
"EndpointID": "c750b0d9d6ae82fec109da15d385b936f79f09bf814dd3b8d03642a2f03d46e2",
"MacAddress": "02:42:ac:1d:00:02",
"IPv4Address": "172.29.0.2/16",
"IPv6Address": ""
}
},
"Options": {},
"Labels": {}
}
]