Take a purely virtual bridge (to be used for VMs) with a static network
configuration:

# cat /etc/sysconfig/network/ifcfg-br0
STARTMODE='auto'
BOOTPROTO='static'
BRIDGE='yes'
BRIDGE_PORTS=''
BRIDGE_FORWARDDELAY='0'
BRIDGE_STP='off'
IPADDR='172.16.1.1/24'

enp0s25 is the local, normal ethernet interface which is working fine:
2: enp0s25: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state
UP mode DEFAULT group default qlen 1000

Now:

# ifup br0
# ip link
2: enp0s25: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master
br0 state UP mode DEFAULT group default qlen 1000
    link/ether 28:d2:44:df:33:18 brd ff:ff:ff:ff:ff:ff
9: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode
DEFAULT group default qlen 1000
    link/ether 28:d2:44:df:33:18 brd ff:ff:ff:ff:ff:ff

It seems that the scripts use the ethernet interface as the *default* value for
BRIDGE_PORTS, which obviously then breaks networking. (If I had wanted a slaved
network device, I'd have configured one.)

brctl addbr br0 ; ip a add dev br0 172.16.1.1/24 ; ip link set br0 up

creates the desired configuration just fine and is what I'd have expected given
the configuration file.