Dynamic IPv6 prefix, old prefix not deprecated (by some clients)

Installing and Using OpenWrt
8

This only shows the advertised valid and pref times of the prefix. I can not view the values for the old prefix, which is still listed in settings under used IPv6 addresses.

For now i am using:

config dhcp 'lan'
	option interface 'lan'
	option start '100'
	option limit '150'
	option leasetime '12h'
	option dhcpv4 'server'
	option ra 'server'
	list ra_flags 'other-config'
	option ra_mininterval '30'
	option ra_maxinterval '100'
	option ra_lifetime '300'
	option ra_useleasetime '1'

This gets me a valid and pref time of 43200 s for the prefix. If i set option leasetime to a lower value. The valid time of the prefix is reduced, but the pref time stays 43200 s, which results in the devices not acquiring an IPv6-address at all.