bug: iOS back transitions between <ion-tabs> pages not animating

# Bug Report

**Ionic version:**

[x] **4.3.1**

**Current behavior:**

Demo app confirgured with { mode: 'ios' } and has 2 <ion-tabs> pages. Navigating forward from Tabs A to Tabs B pages animates as expected. Navigating back from Tabs B to Tabs A does not animate. Removing the { mode: 'ios' } config line shows Android style animations working correctly forwards and back.

**Expected behavior:**

Navigating back from Tabs B to Tabs A page should animate.

**Steps to reproduce:**

- Create a new app
- Change app.module to run in iOS mode
- Create 2 pages, and navigate forward and back between each one

**Related code:**

Basic demo is available at: https://github.com/benmarsh/ionic-multi-tabs-test

**Other information:**

**Ionic info:**

```
Ionic:

ionic (Ionic CLI) : 4.12.0 (/usr/local/lib/node_modules/ionic)
Ionic Framework : @ionic/angular 4.3.1
@angular-devkit/build-angular : 0.13.8
@angular-devkit/schematics : 7.3.8
@angular/cli : 7.3.8
@ionic/angular-toolkit : 1.5.1

System:

NodeJS : v11.12.0 (/usr/local/Cellar/node/11.12.0/bin/node)
npm : 6.7.0
OS : macOS Mojave

```

Start by reproducing the issue with the linked basic demo app, using app.module to enable iOS mode and two ion-tabs pages. Compare forward and back navigation, then trace the ion-tabs transition entry points. Done means navigating back from Tabs B to Tabs A animates in iOS mode without breaking the existing forward animation.

由索引模型根据 Issue 内容生成。