Thank you for reaching out to us, @RTP Engineers.
It's possible your new admin's account was created after October 2021, which is why he gets a new UI when creating an account. It may also be that your new admin is in Business View. I suggest switching to the Accountant View to see if they can select the parent account.

If switching won't make difference, let's perform basic troubleshooting steps because the system occasionally behaves strangely when the browser frequently accesses pages. I recommend using an incognito window to access your QuickBooks Online (QBO). The keyboard shortcuts are as follows:
- Mozilla Firefox or Microsoft Edge: CTRL + Shift + P
- Google Chrome: CTRL + Shift + N
- Safari: Command + Shift + N
Additionally, let's clear the cache on your browser to start fresh or use another supported web engine to obtain the finest and most secure experience with QBO.
You can use these articles as reference to manage your accounts in QuickBooks Online:
Leave a comment below if you have follow-up concerns regarding your accounts and preferences in QuickBooks Online. I'll be around to help. Keep safe.