Excerpt from RFC6204 / RFC7084:
Basic Requirements > for IPv6 Customer Edge Routers
4. Requirements
4.1. General Requirements
G-5: By default, if the IPv6 CE router is an advertising router and
loses its IPv6 default router(s) and/or > detects loss of
** connectivity on the WAN interface**> , it MUST explicitly
invalidate itself as an IPv6 default router on each of its
advertising interfaces by > immediately transmitting one or more
** Router Advertisement messages with the “Router Lifetime” field**
** set to zero** > [RFC4861].
4.3. LAN-Side Configuration
L-13: > If the delegated prefix changes> , i.e., the current prefix is
replaced with a new prefix without any overlapping time
period, then the IPv6 CE > router MUST immediately advertise the
** old prefix with a Preferred Lifetime of zero** > > and a Valid
** Lifetime of either a) zero** > or b) the lower of the current
Valid Lifetime and two hours (which must be decremented in
real time) in a Router Advertisement message as described in
Section 5.5.3, (e) of [RFC4862].
Especially Windows clients can’t handle dead IPv6 prefixes which still have any valid or preferred lifetime left. I know, I can use a script, but I don’t like scripts.