I am playing with the Stripe API and creating “connected accounts” which is Stripe lingo for a sub-account of sorts.
I noticed that the Stripe API from 2018-05-21 returns the test or live API keys after calling stripe.Account.create() depending on whether you used test or live keys when calling the method. If you set a live key on stripe.api_key you get live keys returned when calling stripe.Account.create(), etc.
But the “keys” key on the returned JSON does not exist in the 2020-08-27 version of the API (perhaps earlier versions too).
It appears that you should use the platform’s keys for newly created connected accounts. And you should use your publishable keys, when necessary, for "connected accounts" created under the platform.