BeforeRouteLeave stops working after mixin merge?

Question

Vue.js

There is a mixin (part of it):

beforeRouteLeave (to, from, next) {
    this.$store.commit('ui/SET_LEAVING', true)
    this.routeNext = next
  }

After applying it in the component file, everything works fine (I run it later routeNextin the watcher). But! If I add in addition to the mixin additionally beforeRouteLeavein the same component, then everything stops working. It turns out that Vue does not merge beforeRouteLeave?