Your MX is receiving an IPv6 address on the WAN because the ISP is distributing one via DHCP6.
There is no way to disable the MX from receiving this address.
If you believe it's causing an issue, contact your ISP and ask that they disable IPv6 on the link. Or you could try contacting Meraki support and see if they can disable it on the MX from the backend (not sure if this is possible or not)