Multiple account with same email collapse confusion

March 17 I got the following email, mentioning account A in the subject line:

Hello,

You are receiving this message because we have identified that you are currently using the same email address for this AWS account (as listed in the Subject line) and for additional AWS account(s), which are associated with your Amazon.com account. We strongly recommend that you update the root user email address [1] for this AWS account as soon as possible to separate access to your additional AWS account(s) linked to your Amazon.com account. If you do not take any action by April 10, 2023, we will require you to update your email before accessing this AWS account when you sign in next to your account.

After you have changed the root user email address for this account, you will be able to use it to access your account. At that point, we can finish separating your additional AWS account(s) from your Amazon.com account.

The following are your additional AWS account(s) linked to your Amazon.com account:

<account B>

I changed the root email on account A and that all works fine.

On logging into AWS with the original email I would expect to end up in account B. Logging in seems to work fine, but it just leads to this page:

It if it’s offering to sign me up, it seems like there isn’t actually an AWS account there, but the original email implies there is and gives the account number!

I’ve tried with support, but of course it reads like I’m trying to break into account B, so all they do is ask me to reset the password on account B, which isn’t the issue!

Any ideas how I can determine whether or not account B actually exists and/or close it?