Gets stranger......
I contacted GP service on the website form and got the reply

I am sorry for the inconvenience that you have experienced. Recently,
you received a Platinum Extra Certificate with incorrect member
information. In an effort to ensure the security of your Hyatt Gold
Passport account, we have placed your membership account on hold and
will be providing you with a new account number. We will ensure all
your information and membership activity are unaffected by this changed.

But I haven't had or used a Platinum Extra Certificate anytime recently; and it makes no sense saying I need a new account number!
I emailed back to ask when and how this might be fixed, so will see....

So much for making any more Hyatt reservations this week as I was planning though!

Any thoughts, Bill ?