Pinpoint template helpers no longer working?

We are following this guide: https://docs.aws.amazon.com/pinpoint/latest/userguide/message-template-helpers.html

However, it seems that none of these helpers work. I've tried sha256, sha512 and uuid. I get the following render failures from my campaign every time:

Attribute 'sha256' is not present in the rendering data.
Attribute 'sha512' is not present in the rendering data.

Attibute uuid does not give a render failure, but simply doesn't show anything.

Any guidance would be greatly appreciated.