NetworkManager ignores my config

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This is a laptop running Fedora 28. Today NetworkManager started to ignore my configuration settings, which had worked for years, literally.

I want:

• physical interface enp0s25 plugged into bridge br1
• no IP address on enp0s25
• br1 gets IP address via DHCP

Since today, NM doesn't plug enp0s25 into the bridge. It runs DHCP on enp0s25 and sets an IP address. Since br1 is not connected to the outside world, its DHCP fails.

I use this laptop as a VM host. When I start a VM, libvirt plugs its interface (named vnet0 etc) into br1. However now, a few seconds later, NM removes the interface from br1. It didn't behave like this yesterday.

To add insult to injury, NM runs even after I disable it and reboot:

$ systemctl status NetworkManager
● NetworkManager.service - Network Manager
   Loaded: loaded (/usr/lib/systemd/system/NetworkManager.service; disabled; vendor preset: enabled)
   Active: active (running) since Thu 2018-06-21 20:18:22 JST; 2min 23s ago

The journal just reports what NM is doing. It doesn't say that my config is invalid, nor does it give any other reason why it is ignored.

How can I find out where the problem is?

The interface and the bridge are configured as follows. Adding NM_CONTROLLED=no leaves them unconfigured; I can then ifup them manually and attach my VMs to the network as before:

$ cat ifcfg-enp0s25
TYPE=Ethernet
BOOTPROTO=none
NAME=enp0s25
DEVICE=enp0s25
ONBOOT=yes
BRIDGE=br1
$
$ cat ifcfg-br1
TYPE=Bridge
BOOTPROTO=dhcp
DEFROUTE=yes
PEERDNS=yes
PEERROUTES=yes
NAME=br1
DEVICE=br1
ONBOOT=yes

And this is the situation that results from this configuration:

$ brctl show
bridge name     bridge id               STP enabled     interfaces
br1             8000.000000000000       no
$
$ ip a show dev enp0s25
2: enp0s25: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether f0:de:f1:62:b9:f1 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.24/24 brd 192.168.1.255 scope global dynamic noprefixroute enp0s25
       valid_lft 7088sec preferred_lft 7088sec
    inet6 fe80::f2de:f1ff:fe62:b9f1/64 scope link noprefixroute
       valid_lft forever preferred_lft forever
$
$ ip a show dev br1
4: br1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
    link/ether de:db:4a:cd:8b:96 brd ff:ff:ff:ff:ff:ff