[Ionic 4] ion-content full screen still shows a bit white background on top of the page

I set the ion-content as fullscreen and make the toolbar have transparent background with this css:

And here’s the result i get:

The white part does not appear every time. It disappears when i refresh the page (on browser) or go to another page and go back to this page. Does someone have any idea about this problem and how can i fix it?