I am also facing the same issue. If the flex connection loosing the vlan's then how it integrates with WLC. i have 5 Aps in my network. i have seen the same issue at the time of the AP integrated with the WLC. Out of 5 APs are integrated with WLC and remaining 2 APs are rebooting.
if we loos the flex vlan mapping the how those 3 APs are integrated.