Who the hell is giving their Roku a public IP? How is this remotely a problem? This is 100% not the fault of Roku, and 100% the fault of the ISP. The ISP should have an IPv6 to IPv4 gateway built into their modem/router. You have a WAN port that is IPv6 and an LAN port that IPv4. IPv6 for local networks, makes no sense is completely unnecessary, and is a hill I will die on. IPv4 is here to stay.
Why would you make everything gratuitously complicated by having two separate forms of addressing? All that IPv4 gains you is new and exciting ways to mess up your networking. Just give every device a normal public address (of course you probably want to firewall off inbound traffic from the WAN to the LAN, but that's got nothing to do with addresses) and have a normal network rather than some bizzare frankenstein mashup.