First, let me say that I built this (I ignored router D, and just did an A-B-C triangle), and it worked fine. Obviously, this topology is different from the first one you asked about, so let’s keep our references the same from here out.
I am suspicious of your IPv6 global addresses. Here’s a break down from your output above:
A:
2001:DB8:3C4D:1:240:BFF:FEEA:9201
B:
2001:DB8:3C4D:1:230:A3FF:FEA1:4201
C
0/0
2001:DB8:3C4D:1::1/64
0/1
2001:DB8:3C4D:2::1/64
It appears to me that A and B both have 2001 addresses on the same /64 subnet, while C is using addresses on different subnets on Gig0/0 and Gig0/1. You see how Gig0/1 is using :2::/64, while everything else is using :1::/64? What’s going on there?
When A wants to contact B, it will believe it is on the same subnet, so it will try to do neighbor discovery on its own network segment instead of going through its gateway (router C).