Despite having preemption disabled, does the first controller regain "master" status of the VRRP when coming back online?
You can also shutdown the VRRP intances on the master (rather than rebooting) to test this failover.
config t
vrrp <id>
shut
show vrrp (verify it is in INT state)
verify that the other controller is master for this IP
show vrrp (verify it is Master)
bring the master back online
config t
vrrp <id>
no shut
show vrrp (verify status; should stay Backup)
Also, can you confirm what you have set for your primary/backup LMS in the AP System profile for those APs?