Do config vars persist after a failed Heroku dyno or stack upgrade?

The goal is to determine whether Heroku’s automatic recovery after a failed dyno or stack upgrade preserves environment variables and add‑on attachments.

Uncertainty exists regarding the platform’s rollback behavior: does the release rollback guarantee that all config vars remain unchanged, or can they be reset during a failed upgrade?

Which upgrade scenarios (stack change vs dyno class) most commonly affect configuration persistence, and are there documented safeguards to ensure config integrity during recovery?