I did some tests, and that's what I saw.
If the wireless device is connected to a wireless network that belongs to vlan 1 (default) and ping another device or server connected to vlan 1 is seamless roaming operates without packet loss or a maximum of 1 packet loss. If the wireless device is connected to a wireless network belonging to any vlan (eg 10) during the ping server of vlan 1 or resource on the Internet packet loss in the seamless roaming can reach 4 package. At the same time when you start to ping a server connected to vlan 1 is the first packet is lost, and then starts to go ping. As if the need some time to do the routing.
I think it is not normal. Routing between vlan configured dynamically NAT. Maybe that's the problem?