Excerpt from RFC6204 / RFC7084:
Basic Requirements for IPv6 Customer Edge Routers
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.