How to get the origin of an unhandled promise rejection in browser?

I'm currently facing an impossible to reproduce bug.

1. My front error logger is Bugsnag and the front use Vue.js / vue router / vue apollo Graphql
2. I'm sometime having errors popping in Bugsnag that say: unhandledrejection handler received a non-error. With that: no file, no stacktrace.
3. the user actions to have this error is when going from one page to another, everytime the same origin and destination, but I can not reproduce in local or in production. I don't know if the user is blocked or if the error is silent to him
4. I've tried to use https://github.com/grantila/trace-unhandled to log more informations, but the only things I get is:

UnhandledPromiseRejectionWarning
[ Stacktrace altered by https://github.com/grantila/trace-unhandled ]
Unknown error
    ==== Promise at: ==================


    ==== Error at: ====================


    ==== Shared trace: ================

But when creating my own unhandled rejected promise I can get something like that:

UnhandledPromiseRejectionWarning
[ Stacktrace altered by https://github.com/grantila/trace-unhandled ]
Unknown error
    ==== Promise at: ==================
    at eval (webpack-internal:///./node_modules/cache-loader/dist/cjs.js?!./node_modules/babel-loader/lib/index.js!./node_modules/vuetify-loader/lib/loader.js?!./node_modules/cache-loader/dist/cjs.js?!./node_modules/vue-loader/lib/index.js?!./src/components/pages/Customer/CurrentCases.vue?vue&type=script&lang=js&:127:9)
    at Module../node_modules/cache-loader/dist/cjs.js?!./node_modules/babel-loader/lib/index.js!./node_modules/vuetify-loader/lib/loader.js?!./node_modules/cache-loader/dist/cjs.js?!./node_modules/vue-loader/lib/index.js?!./src/components/pages/Customer/CurrentCases.vue?vue&type=script&lang=js& (http://....../js/23.js:23:1)
    at __webpack_require__ (http://....../js/app.js:790:30)
    at fn (http://....../js/app.js:151:20)
    at eval (webpack-internal:///./src/components/pages/Customer/CurrentCases.vue?vue&type=script&lang=js&:2:339)
    at Module../src/components/pages/Customer/CurrentCases.vue?vue&type=script&lang=js& (http://....../js/23.js:107:1)
    at __webpack_require__ (http://....../js/app.js:790:30)
    at fn (http://....../js/app.js:151:20)
    at eval (webpack-internal:///./src/components/pages/Customer/CurrentCases.vue:3:100)
    at Module../src/components/pages/Customer/CurrentCases.vue (http://....../js/23.js:95:1)
    at __webpack_require__ (http://....../js/app.js:790:30)
    at fn (http://....../js/app.js:151:20)

    ==== Error at: ====================


    ==== Shared trace: ================

So, my question is : What can I do to find more about the origin of an hundled promise rejection in browser ?