Ionic V4 ios : Header is dissappearing when keyboard and toaster active same time

I have an issue where the ionic header is disappearing from the screen (like pushed out of the screen to top) when the keyboard is active and then toaster pops up header goes away. this is happening only in ios build. is this because the ios platform keyboard is different? This is happening when the ion-input keyboard is active and on button click form is validating and throwing a toaster if the field is not valid.
I have tried solutions so far:
<ion-header class="header-fixed-position"><ion-toolbar slot="fixed"></ion-toolbar></ion-header>

ionic info:   
Ionic CLI                     : 5.4.1 (/Users/....../node_modules/ionic)
Ionic Framework               : [@Ionic](https://ionic-worldwide.slack.com/team/UAMJBTZST)/angular 4.11.7
@angular-devkit/build-angular : 0.803.26
@angular-devkit/schematics    : 8.1.3
[@yuny](https://ionic-worldwide.slack.com/team/UCF966ZBP)/cli                  : 8.1.3
[@Ionic](https://ionic-worldwide.slack.com/team/UAMJBTZST)/angular-toolkit        : 2.0.0Cordova:   Cordova CLI       : 9.0.0 (cordova-lib@9.0.1)
Cordova Platforms : android 8.1.0, ios 5.1.1
Cordova Plugins   : cordova-plugin-ionic-keyboard 2.2.0, cordova-plugin-ionic-webview 4.1.1, (and 13 other plugins)

why is it happening? I have no idea. I tried adding styles for ion header with position fixed and slot=fixed is not helping here.
Help would be appreciated.

Anyone who knows why this is happening. i have no luck on this so far.