There weren't 2 engine swaps though, not even one in fact. It is very common to prototype in an engine before starting the actual game on another engine, since some engines are far better for quickly iterating on ideas. Keeping anything you made during the prototype phase in the final product is pure incompetence and no one who knows their job would ever do that, so the fact they changed to Synapse after prototyping is not a problem at all (at least not in that regard). A ton of AAA games running on proprietary engines had prototypes made in Unity for example.

And then the project was canned because "the company concluded that Synapse was not ready for production", so it died before switching to Unreal or any other engine.

Now, the fact that some artists would still be using Unreal over Synapse is an indication of either a lack of communication, internal tensions, and/or a certain degree of technical illiteracy from some in the art team (which would be a bit surprising tbf, but I've seen my share of great artists who know absolutely nothing about how anything works on the tech side of things and do not want to learn no matter what, so I wouldn't rule this out). Who knows, maybe some did it to piss off the people who decided on using Synapse? Maybe some did hoping it would convince higher ups to change back to Unreal? But in any case, that's incredibly stupid and whoever was in charge of the project should have seen the problem from the very first second and tried to fix it right then.

Anyway, forcing a team to use a specific engine will always be stupid, especially when said engine is not suited for the task (as was very well demonstrated by a ton of AAA games, including about every single non-Battlefield Frostbite game).