How could everything from RenderWare have been moved to Unreal but with more bugs?

[GrudgefromSanAndreas]

Trilogy DE has exact same physics, anims, gameplay and almost everything else as OG RW versions and i'm kinda surprised this has worked at all. maybe it's not just gsg's fault, maybe renderware has some actual incompatibilites with unreal engine. i really dont understand how transferring everything from one game engine to another works and this just has blown my mind to this day

[MeatSafeMurderer]

Simple. Under the hood it's kinda not Unreal at all. There is still a large amount of RenderWare code, with Unreal handling the rendering. That's why many existing bugs from the original games still existed in the "Definitive" Edition at launch. If it were wholly Unreal, there may have still be bugs, but they would've been completely different. That being said, these days it is generally possible to recreate a game entirely in a new engine without necessarily reusing the old code...but you have to recreate it from the ground up. Nothing can be reused as is.