Please don't merge this yet, as it seems it might not fully solve the issue

@edusperoni @NathanaelA
When debugging this issue in the use case Root => A => B, then back to A it looks like all keys to store/retrieve are generated correctly (also without this fix), but the error is still thrown.
The expression that causes the error to be thrown (curr.value.routeConfig !== result.value.routeConfig in https://github.com/angular/angular/blob/3817e5f1dfeb9de26fa2ea4068a37565f435214f/packages/router/src/create_router_state.ts#L52) will actually show equal properties and values when inspecting, but are not the same object reference. This only occurs with lazy loaded modules (if changed to "regular" route configs it works).
Perhaps the solution direction for this part of the issue is to either use a "deepEqual" instead of reference equality or to somehow make sure that the routeConfig reference stays the same for lazy loaded modules.
Hope this helps, cheers.