BusyIndicator doesn't animate

In past versions of Qt, I could create a busy indicator just before a long operation started and have it animate until the operation completed. In Qt 5.15.7, the BusyIndicator doesn't animate anymore. If I delay the start of the long operation for a couple of milliseconds after the BusyIndicator's running is set true, then the indicator's animation starts and then stops soon after the busy operation kicks off.

I tried replacing BusyIndicator with an icon of my own and used RotationAnimator on that icon. Same issue there as well. It appears both BusyIndicator and Animator are broken. They don't seem to update the UI from the render-thread.