Thank you for Subscribing email error on First Name

I can't find this addressed anywhere else, so maybe it is a new issue. I have created a simple custom text email to thank people for subscribing to my list. I created it within the Manage Subscribers section. It opens like this

Hi [First Name,fallback=New Subscriber],

However, the emails arrive at the new subscriber with that exact text appearing in them, no "Hi Steve" and no "Hi New Subscriber"

This variable field syntax works fine in my regular campaign emails, which I love BTW, very easy to use, track etc.What am I doing wrong?

Thanks in advance for responding.

Steve

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