Then Function Running before Store Action in Vue

As mentioned by @deceze:

Adding async and await to:

  async getBrandMerchants({ commit }, id) {
    commit('setLoading', true);
    await BrandService.getBrandMerchants(id)
      .then((response) => {
        commit('setBrand', formatter.deserialize(response.data.result.brand));
        commit('setMerchants', formatter.deserialize(response.data.result.merchants));
        commit('setLoading', false);
        console.log('First');
      })
      .catch((error) => {
        if (error.response.status === 401) {
          dispatch('alert/error', error.response, { root: true });
        } else {
          Toast.open({
            type: 'is-danger', message: error.response.data.meta.message,
          });
        }
      });
  },

and

async addMerchantsToBrandGroup(index) {
      const { id } = this.rows[index].brand;
      if (id === null) { return; }

      await this.getBrandMerchants(id)
      console.log('Second');
      this.rows[index].merchants = this.merchants
    },

Solved the problem. Thanks!