QWebEnginePage scroll position reporting randomly fails

Apparently depending on something during initialization, WebEnginePage scroll position reporting sometimes fails. In this situation scrollPosition() always returns 0,0, and no scrollPositionChanged signals are emitted.

Reproduced on multiple Linux versions and recent Qt versions, for example, Debian bullseye with Qt 5.15.2+dfsg-3, or Arch Linux with 5.15.10-2

It's more or less easy to reproduce depending on ? I usually start the attached program in a loop: while true;do ./wescroll ;done, trying to scroll then exiting the window until it fails.