I've seen all the theories about how it was broken but since the new shadowlands Afterlives cinematic, I'm thinking that Arthas being unjudged by the arbiter is what caused the machine to break. Time is chaotic in the shadowlands, even though it said the machine broke in sometime around Legion(which would be the time we're going off) doesn't mean it that would of been the timing in the shadowlands. The Kyrian are responsible for ferrying the souls of the shadowlands to arbiter and to their destination. Devos and Uther skipped the judging of arthas and sent him into the maw. Now I think the true significance of doing this lies in that this most likely has never been done before. Devos most likely is the first fallen Kyrian( her seeing uther's wounded soul, due to powers of the maw) caused doubt of the kyrian path. I would like to hear other thoughts on this.