bug: SelectedTab on ion-tab-bar does not activate selected tab as default

# Bug Report

**Ionic version:**
[x] **4.x**

**Current behavior:**
Preferred tab as per property `selectedTab` when opening tabs is not selected

**Expected behavior:**
Preferred tab as per property `selectedTab` when opening tabs is selected (as per documentation).

**Steps to reproduce:**
Take sample code as per `https://github.com/ionic-team/ionic-docs/tree/master/src/demos/api/tabs/index.html` and put in a separate file with extension html.

Replace code `` with ``

Open file in Chrome. Tab movies is not selected.

**Related code:**
See above

**Other information:**
Plain javascript file, no framework used. Same issue seems to appear when trying to set `selected="true"` on ion-tab-button.

Related issue reported https://stackoverflow.com/questions/53860450/ionic-4-preselect-a-tab-in-a-modal. Using `selected-tab` as property does not work. Using JS is not desired (through controller).

**Ionic info:**
```
[WARN] You are not in an Ionic project directory. Project context may be missing.

Ionic:

Ionic CLI : 5.4.5

Utility:

cordova-res : not installed
native-run : not installed

System:

NodeJS : v12.13.0
npm : 6.12.0
OS : Windows 10
```