Set up client environment variables and deployed, but not defined on prod?

I've integrated Posthog into my wasp application, but whilst it works locally with my environment variables, it does not work when deployed.
As mentioned in the docs, I am using the correct syntax - eg. import.meta.env.REACT_APP_PUBLIC_POSTHOG_HOST,

I have also made sure to deploy these new secrets to prod, and when requesting them via fly secrets list -a <app name>, they come back.

However, when attempting to initalise posthog on prod, it throws an error saying the API key is undefined