I am experiencing the same issue. Any reservations made since early October are not displaying on Hyatt.com. However, phone agents see all reservations in their system. Modifications to reservations made before early October still display, even when the date is changed.

I first reported this issue to Hyatt on October 5 and then again 10 days ago through my personal line rep who says they took screenshots of what is showing when logged into my account on the website and sent it to Hyatt IT in Chicago. So far nothing has changed and I have had to resort to tracking reservations in my calendar (I have over 100 upcoming reservations).