On top of the problem described in the OP, my ISP, in their infinite wisdom, has decided to break IPv6 by dropping all packets at my edge gateway, if their source addresses come from prefixes announced through Router Advertisements.
This not only broke odhcp6c, but also my Linux and Windows desktop PCs lost IPv6 connectivity by using one of the RA addresses as the system-wide default IPv6 address (this happens when they are connected directly to the bridged cable modem).
Trying to get my ISP to fix this is a waste of time; it's either automated "tech" support or stupid telemarketing types that don't even know what IPv6 is.
For the time being, as a dirty fix that "solves" both issues, I've removed router-advertisement from the list of accepted ICMPv6 types in the Allow-ICMPv6-Input traffic rule. IPv6 address and prefix is obtained solely by a explicit request from odhcp6c (configuration options reqaddress and reqprefix).
If you are from Brazil and is reading this, STAY AWAY from Claro/NET, you will thank me later.