When playing Disco's Evil Santa Boss Fight and died it occurred to me that the death message I got back was "ilxMine was knocked from a high place and got finished off by entity.SmallFireball.name". So the death message showed the java entity name because it is probably not defined well.
It is hard to reproduce but I made a small contraption to let you see how it works. (See attachment). In the world is also a small explanation of how I did it.
I think it's easy to fix.