Comment # 1 on bug 1169476 from Franck Bui

(In reply to pgnd _ from comment #0)
> it's a known issue,

I would call that a limitation instead ;)

As you already noticed this limitation has been removed in the last versions of
systemd (since v242) but the version used by Leap 15.* is much older and thus
systemd-networkd stuff doesn't support waiting for bridged interfaces.

Backporting this feature is not trivial and is not planned for the time being
especially since networkd is not the default network manager used by Leap/SLE
distros, sorry.

Hope that make sense.