The Start of AnvilNext
AnvilNext developed by Ubisoft has been in use since the Assassins Creed: Unity - decidedly one the best looking games they have released in that generation. The ground-breaking lighting, massive streaming worlds with multiplayer support, the team threw everything they had at that initial engine, the game might not have been a commerical success for them, but it was a starting point which lead to success in future development.
Assassin's Creed Development
Assassin's Creed: Origins & Assassin's Creed: Odyssey are two great titles both made with the AnvilNext engine, and while it's hard to believe at Ubisoft they have lots of game engines running concurrently in different development pipelines. Rather than stick with one engine, they maintain a massive amount of code for ease of use with specific projects.
Assassin's Creed has become synonymous with using AnvilNext, Far Cry uses their Dunia Engine, Division uses the Snowdrop Engine (with visual scripting for artists). I would have thought it would make sense to go with Snowdrop for Ghost Recon: Breakpoint since they could build upon the third person shooter framework they had there, however it seems like they wanted the animation, physics system that was built in Assassins Creed for use with motion matching technology they have been developing.