Rails 6 app not reading ENV on Heroku during tests

I have a Rails 6 app deployed to Heroku and working fine. I've just started adding an API to the app, and this is also working fine locally, with all tests passing locally.

When I deploy to Heroku, the app responds correctly to curl.

However, my tests are failing with TypeError: no implicit conversion of nil into String during authorisation of the JWT token. I've identified the root cause, which is that the DEVISE_JWT_SECRET_KEY environment variable isn't being loaded for my tests.

This variable is defined as a Heroku environment variable, and works fine when I test the actual app using curl, as I mentioned. Running

heroku config:get DEVISE_JWT_SECRET_KEY -a [APP_NAME]

returns the correct key.

When I add a line to my tests to check for the variable, the raise is executed when the tests run, showing that the variable is missing.

raise if ENV['DEVISE_JWT_SECRET_KEY'].nil?

Can anyone suggest why environment variables aren't loading when I run my tests on Heroku?