Last night I tried the following-

/64 on vlan40 (DMZ)

/64 on vlan50 (Private RFC1918)

in IPV6 -> Addresses on the Miktotik.

Assigned a static IPV6 address on a host on vlan40. It would work for a few minutes and then IPV6 connectivity would disappear. Did some more playing around and found it would also do this when using auto assignment.

Restored it back to just the /64 on vlan40 and it seems to work again.

Anyone know what's going on?