TYPO3 Core

"TypoScript overrides" does not work when using the render ViewHelper. That's also mentioned within the docs. The corresponding code is only part of the FormFrontendController. I guess it should be possible to move the code to FormPersistenceManager where I would expect the code. That should make it possible to always apply the TypoScript overrides, no matter how the form is retrieved.

Regarding backend context: TypoScript should be available via module.tx_form.
So if one configured the overrides in both plugin as well as module it should work in both places by using Extbase configuration manager, which should already respect the current TYPO3 context.