Taro.navigateBack({delta:1}); 有时候触发了不返回

**问题描述**
首页跳到二级页面 然后点返回 有时候返回不了 但是再进入三级页面 返回二级页面 再返回就可以了

```js
Taro.navigateBack({delta:1});
```

**期望行为**
正常返回

**报错信息**
无报错

**系统信息**
Taro CLI 2.0.4 environment info:
System:
OS: macOS 10.15.7
Shell: 5.7.1 - /bin/zsh
Binaries:
Node: 10.16.1 - /usr/local/bin/node
Yarn: 1.17.3 - /usr/local/bin/yarn
npm: 6.14.4 - /usr/local/bin/npm
npmPackages:
@tarojs/async-await: 3.0.14 => 2.0.4
@tarojs/components: 3.0.14 => 2.0.4
@tarojs/mini-runner: 3.0.14 => 2.0.4
@tarojs/router: 3.0.14 => 2.0.4
@tarojs/taro: 3.0.14 => 2.0.4
@tarojs/taro-alipay: 3.0.14 => 2.0.4
@tarojs/taro-h5: 3.0.14 => 2.0.4
@tarojs/taro-qq: 3.0.14 => 2.0.4
@tarojs/taro-quickapp: 3.0.14 => 2.0.4
@tarojs/taro-swan: 3.0.14 => 2.0.4
@tarojs/taro-tt: 3.0.14 => 2.0.4
@tarojs/taro-weapp: 3.0.14 => 2.0.4
@tarojs/webpack-runner: 3.0.14 => 2.0.4
eslint-config-taro: 3.0.14 => 2.0.4
eslint-plugin-taro: 3.0.14 => 2.0.4
nerv-devtools: ^1.5.7 => 1.5.7
nervjs: ^1.5.7 => 1.5.7
react: ^17.0.1 => 17.0.1
stylelint-config-taro-rn: 3.0.14 => 2.0.4
stylelint-taro-rn: 3.0.14 => 2.0.4