Hello Jonn,
This is a known IOS limitation. To be honest, I do not know if there is any sensible explanation for this - it is just implemented this way. If it is allowed to use the "deny" clauses in the ACL you can perhaps solve your problem by using a /127 multicast group prefix and denying the unwished multicast group address. Otherwise, I believe it is not possible to define an IPv6 RP for a single multicast group in Cisco IOS.
Best regards,
Peter
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