Finally managed to reach an agent on the US 800 line. It's a relief to know that my award ticket (using CX miles) on 5/23 is still good. The agent was kind enough to try the "manage booking" tab on the JAL site, and he had the exact same problem on his end. After several minutes of trying, he still couldn't explain the reasons or pinpoint the cause. According to him, there's zero "transaction history" since the creation of my ticket, i.e., no schedule or other changes at all throughout for my reservation.

JAL, please fix the MB tab!