We set up a new point to point GRE tunnel between R1 and R2 running OSPFv3 and IPv6. This was a second tunnel on R1 that mirrored the other GRE tunnel.
Now, although we were able to ping each other's source interfaces, ping FF02::5 and had PIM neighborship, OSPF never came up. We saw that both R1 and R2 were each sending Hello packets, but neither end was receiving them. My other tunnel was fully up throughout this time.
Throughout the day, along with troubleshooting, we periodically deleted and recreated the tunnel. For some reason, maybe after a "clear ospf process", after we recreated the tunnel one more time it finally came up.
The question I have: what could have caused this issue and why didn't one of the prior attempts to recreate the tunnel work?