Why is stateMapping.stateToRoute() called after a UI refresh triggered by the popstate event?

[JJWesterkamp]

Using the history router, I'm having trouble getting the right state in the URL after popstate events. To show you what I mean I created logging of the data that goes in stateMapping.stateToRoute() and in stateMapping.routeToState() and also their return values on each invocation with a method decorator (the factory of which I call traced):

Note: I'm using the vanilla JS instantsearch.js library. This is not Angular, although it might look like it :)

import { RouteState, StateMapping, UiState } from 'instantsearch.js/es/types'

export class StateMapperComponent implements StateMapping {

    @traced(routeToStateTrace)
    public routeToState(routeState: RouteState): UiState {
        // ...
    }

    @traced(stateToRouteTrace)
    public stateToRoute(uiState: UiState): RouteState {
        // ...
    }
}

This results in a log such as in the following image:

I have used this to see whether the states are converted properly, and in all cases both my implementations of routeToState and stateToRoute work as they should.

Problem:

After clicking the browser back-button the popstate event fires, and that triggers, as expected, a uiState update, routeToState is called, and my log shows that the new UI state returned by it is from my view correct. But shortly after that also stateToRoute is called. See below screenshot of this event:

This screenshot shows that the UiState that is used as input for the new RouteState does not match the UiState generated by routeToState before, resulting in the wrong URL for the current UI state, probably because the RouteState that is generated from it does pass an equality test against the currently active RouteState. To be specific, the UiState used to generate the new RouteState should not have the sortBy setting, as the UiState generated from the source RouteState shows.

Question 1
Should the history router get a new state to process in the first place after a popstate event? As far as I understand, the route state in the new 'current' history entry should be the source of truth, and it already is the new state for the application.

Question 2
Why do I see that the UiStates don't match? Is this a bug? And if not, what am I doing wrong / misunderstanding?

Thanks in advance!

Scenario
I have multiple refinement lists, all mounted with connectRefinementList, one sortBy widget, one connectRange connector, one hits component mounted with connectHits, one clearRefinements widget, and two pagination templates mounted with connectPagination. I'm using the history router with custom implementations for both createURL and parseURL. They are merely used to transform strings to numbers and vice versa; they don't modify the general structure of the parsed query object.