Firebase functions config won't deploy

I'm trying to deploy a function that uses a firebase config variable based on the environment variable I set before deploying. This works well on staging (a separate project that looks almost identical to production, except for a couple of env vars), but is failing on production with error TypeError: Cannot read property 'env' of undefined pointing to functions.config().app.env (see code below).

If I browse the function online in https://console.cloud.google.com/functions/details/us-central1/<functionName>?project=<projectName> the .runtimeconfig.json file appears to not have picked up the env variable I have set before deploying to production, while it does that perfectly on staging.

If I set app.env to "staging", it works great for my staging environment, and doesn't show any errors. I tried cd-ing into the functions folder to set the env variable, then back out before deploy, but it didn't change a thing. I even tried writing it into .runtimeconfig.json in the functions folder (though that should only be needed for local testing), but that had no effect either.

EDIT: following some more tests it came out that the .runtimeconfig I can see when browsing the function code online is generated from what I set locally via CLI to functions config, and is uploaded when I deploy. It appears this copies fine, whenever I update it, onto staging, but does not do the same into production. I just can't figure out why...

My function:

const functions = require('firebase-functions')
// Stripe needs to be set with its key when the library is being required
const stripe = require("stripe")(functions.config().app.env === 'staging' ? functions.config().stripe.key.staging : functions.config().stripe.key.production)
//                                                      ^^^ this is what the error is pointing to

module.exports = functions.https.onCall(async (data, context) => {
    // ... doin' some stripe magic
})

I'm using npm in a shell command line environment:

firebase functions:config:set app.env="production" && firebase deploy -P production --only functions

Output of firebase use:

% firebase use                                                      
Active Project: default (<stagingProjectName>)

Project aliases for <projectFolderPath>:

* default (<stagingProjectName>)
  production (<productionProjectName>)

Run firebase use --add to define a new project alias.

Output of firebase functions:config:get (among other vars):

{
  "app": {
    "env": "production"
  }
}