This might just be me misreading the messages from the logs, but on my subs it looked like H (The Talon) was unlocked from F (Central Blue), not from G (South Indigo Deep 2). I reviewed the voyage logs, but for some reason it didn't save or display the zone unlocks, and I didn't screenshot them.

Again, I might be entirely wrong with how I read the log message, but can anyone else shed light on this?

(Smirny, I apologize for casting doubt on your info! Everything else you posted lines up with my observations, and thank you for the information!)