There is a bug for the Japanese helicopter tree where the 2005 and later attack helicopter tech are still pre-designer, and so they are unable to be researched because the tech required doesn't exist. They also do not unlock any new chassis/air frames or any new modules. The transport helicopters line works perfectly fine though.

I just wanted to report this here since I do not have a reddit account and I don't feel like giving discord my phone number just to join a server for a random mod, feel free to report this issue on the discord or subreddit.