At long last, Telltale Games is going to ditch their own in-house engine that has been used for a bit over 30 games at this point. They will begin to use the Unity engine for all games after The Walking Dead: The Final Season.

This means that the Telltale adventure game based on Stranger Things should be the first game built using Unity. While many may argue that Unity on consoles isn't all that great, they should agree that it's better than continuing to use Telltale's old engine. This is an engine that has resulted in a number of bugs and issues for fans over the past decade of game releases from Telltale.

This is all speculation at this point but speculation rooted with some evidence as Variety points out.
The studio is shifting to Unity for its projects (both detailed by Variety’s source and referenced in job listings), with the final season of “The Walking Dead” the last to premiere on the old engine. “Stranger Things” is slated to be the first Unity engine project, according to multiple sources.(...)

Sources tell Variety that while the move away from Telltale Tool is long overdue, a new engine means growing pains. Developers are re-learning how to manage workflow as they adapt to the new engine. This is causing a longer process than Telltale is used to working.

The Walking Dead: The Final Season will be released on August 14. Telltale's Stranger Things project does not yet have a release date. All of this also comes just a couple of weeks after news broke that Telltale's former CEO is suing the company that he helped to create.