Invalid painting on widget

I'm using a QTabWidget to show multiple tabs with content. Loading of content can take a lot of time so I've written custom process indicator (in an example, it's class ProcessIndicatorWidget). If you open a tab with QWebEngineView and then subsequently open another tab by clicking on Run Task, you'll see that the run task widget / tab does not update its contents (no update / paint). If a tab is switched back and forth, the widget will be correctly redrawn.

Steps to reproduce:

1. Click button "Open Web Page"
2. Click button "Run task"