on a server running,

    lsb_release -rd
        Description:    openSUSE Leap 15.1
        Release:        15.1

with 15.1's updated

    systemd-234-lp151.26.13.1.x86_64

with systemd-network set to manage network

    networkctl
        IDX LINK             TYPE               OPERATIONAL SETUP
          1 lo               loopback           carrier     unmanaged
          2 enp11s0          ether              carrier     configuring
          3 br0              ether              routable    configured

        3 links listed.

where

    cat /etc/systemd/network/br0.netdev 
        [NetDev]
        Name=br0
        Kind=bridge

    cat /etc/systemd/network/br0.network
        [Match]
        Name=br0

        [Network]
        Address=10.0.0.10/24
        Gateway=10.0.0.1
        IPForward=1
        IPv6AcceptRA=0

    cat /etc/systemd/network/uplink.network
        [Match]
        Name=enp11s0

        [Network]
        Bridge=br0

if a systemd unit is created with a legitimate wait-online dependency on the
bridge,

    cat /etc/systemd/system/some.service 
        [Unit]
        ...

        [Service]
        ...
        ExecStartPre=/usr/lib/systemd/systemd-networkd-wait-online
--interface=br0
        ExecStart=...
        ...

        [Install]
        WantedBy=multi-user.target

on boot, the 'systemd-networkd-wait-online.service' fails

    [  146.400104] systemd-networkd-wait-online[1381]: Event loop failed:
Connection timed out
    [  146.420204] systemd[1]: systemd-networkd-wait-online.service: Main
process exited, code=exited, status=1/FAILURE
    [  146.422071] systemd[1]: Failed to start Wait for Network to be
Configured.
    [  146.422242] systemd[1]: systemd-networkd-wait-online.service: Unit
entered failed state.
    [  146.422268] systemd[1]: systemd-networkd-wait-online.service: Failed
with result 'exit-code'.

it's a known issue,

    systemd-networkd-wait-online fails with bridged interfaces
     https://github.com/systemd/systemd/issues/2154

apparently fixed by bump to systemd version >= 242

    https://github.com/systemd/systemd/issues/2154#issuecomment-536214625