Where it seems you can not configure Mikrotik to service both DHCPv6 address requests and DHCPv6-PD requests on the same interface.
Apparently, this is needed to do DOCSIS 3 (cable) IPv6 support as it does not support assigning IPv6 WAN addresses to routers with Router Advertisements.
A workaround is proxying the DHCPv6 requests to some other sever, like ISC DHCP. That works (both address and delegation requests are served), but there is what I would almost call a bug: when a prefix delegation is sent, the mikrotik proxy does not add a route for the delegated block. Manually adding a route makes everything work until the next delegation.
An additional reason I need this as proxy support is that cable DHCP requires a bunch of custom options -- things I don't see how to add for DHCPv6 in mikrotik. So I must proxy it to something that can handle those options.