Recommendations

Note the following with respect to IPv6 discovery in FortiWLC 8.6.0.

Use only one prefix in RA advertisements.
When using DHCP, the prefix lifetime should be high to avoid IP changes.
The DHCP and default gateway must be the same in a pure IPv6 environment.
Use a prefix-based/stateless configuration when FortiGate is used as the IPv6 DHCP server for APs.
IPv6, Override Default DHCP Server Flag, and DHCP IPv6 Relay Pass-Through should be enabled on the VLAN interface for IPv6 bridging (pass-through) to work.
Enable neighbor discovery optimization (Configuration > Devices > Controller > IPv6 Parameters).

Note the following with respect to IPv6 discovery in FortiWLC 8.6.0.

Use only one prefix in RA advertisements.
When using DHCP, the prefix lifetime should be high to avoid IP changes.
The DHCP and default gateway must be the same in a pure IPv6 environment.
Use a prefix-based/stateless configuration when FortiGate is used as the IPv6 DHCP server for APs.
IPv6, Override Default DHCP Server Flag, and DHCP IPv6 Relay Pass-Through should be enabled on the VLAN interface for IPv6 bridging (pass-through) to work.
Enable neighbor discovery optimization (Configuration > Devices > Controller > IPv6 Parameters).