There are likely a number of moving pieces to that so more config context might help.
Out of curiosity, do you have both a v4 and v6 stack configured on the routers and BGP is carrying both address families (over a v4 peering)?
If the routers don’t have a v6 stack configured, the only BGP mechanism I know for providing BGP signaled v6 over an encapsulation would be 6PE (delivers v6 to v4 PEs via MPLS) or EVPN (delivers v6 to v4 VTEP/PE via VXLAN/MPLS).
Conversely if you only have a v6 stack configured, you’d need RFC5549 support to deliver v4 over v6, again assuming MPLS.
To my knowledge, none of that is actually supported by Mikrotik at the moment, although at least 5549 support is on the radar [1].
I think the safest answer right now is dual stack.
If you do have dual stack configured and it’s still not generating the correct next-hops for each address family, then there might be something odd going on.
More context about which of those you’re trying to do would be helpful. (Also how is VXLAN involved in this situation?)
[1]
https://help.mikrotik.com/docs/display/ ... l+Overview