It depends on what your configuration was previously, it may be incorrect and just happened to work accidentally.
In particular DHCPv6 has no mechanism to obtain or provide a default gateway. The Mikrotik DHCPv6 client add-default-route=yes is a hacky bodge, it uses the address of the DHCPv6 server from which the address/prefix/other information was received - this works if the DHCPv6 server and the default gateway are the same host, but otherwise fails so in general set add-default-route=no.
If the WAN connection is IPoE the correct method is to use received router advertisments (RA) which unfortunately are not displayed by RouterOS, as discussed in other forum posts, and was completely broken in earler releases of RouterOS v7. The default IPv6 settings include forward=yes and accept-router-advertisements=yes-if-forwarding-disabled, so as you are forwarding you need to set accept-router-advertisements=yes.
If the WAN connection is PPPoE you can either set the PPPoE client add-default-route=yes, as this works on point-to-point links, or use router advertisments.