Hi Derek,
as follows from your post, the coordinator doesn't see the router 0x7B40 as a neighbor but the router 0x7B40 does see the coordinator (...coordinator shows 0x7B40 with inc cost of 0x0, outgoing cost of 0x0, ..... Link Status from 0x7B40 show coord incoming cost 0x1, outgoing 0x0).
Are you sure that after the restart of the router 0x819A, the router 0x7B40 gets MAC acks from the coordinator and not from the the router 0x819A (0xFAD5)?
Regards,
Ilya