@ranjankumarsgh, there are only two settings for IPv6 on the MR, that’s auto and static - assuming you are using a firmware version that supports IPv6 (which you obviously are). And I’m not aware of a way of disabling it, but it shouldn’t cause you any problems.
When an access point comes up it will create a link local IPv6, check for uniqueness, and then send a router solicitation to determine its global configuration details. I would suggest that for the access point that is getting an IPv6 address assigned it is communicating with an IPv6 enabled router (i.e. it’s responding with a router advertisement). Incidentally it’s responding with a site-local address (fec0::/10), a standard that has been deprecated in favour of Unique Local Addressing.
I think if you find the router that’s issuing IPv6 RAs and disable IPv6 on that, then you’ll find that the MR will no longer auto configure an IPv6 address.