Weird behavior with redux not updating store state

Question

I just ran into an issue, with my redux store not updating after a matched action is dispatched.

I managed to get my reducer to work, here it is:

// working approach

import { AnyAction } from 'redux';

export interface IReducerState {
    config: (Record<string, any> & {
        name: string;
        age: string;
    }) | null
}

const initialState = {
    config: null,
};

const reducer = (state: IReducerState = initialState, action: AnyAction): IReducerState => {
    const { type, payload } = action;

    switch (type) {
        case 'MATCHED_ACTION':
            return {
                ...state,
                config: {
                    ...state.config,
                    name: payload.clear ? '' : state.config.name,
                    age: payload.clear ? '' : state.config.age,
                    [payload.field]: payload.value,
                },
            };

        default:
            return { ...state };
    }
};

export default reducer;

But before, however, I was doing the following:

// alternative approach

case 'MATCHED_ACTION':
    const result = { ...state.config };

    if (payload.clear) {
        result.name = '';
        result.age = '';
    }

    if (payload.field && payload.value) {
        result[payload.field] = payload.value;
    }

    return {
        ...state,
        config: result
    };

What am I doing wrong? Why is the alternative approach incorrect?