It reproduces this problem stably on macOS but not on Windows. It may be related to QTBUG-116301.

step.1

open one or more dialogs in mainWindow

step.2

open QFileDialog in dialog

step.3

When one QFileDialog is opened, mainWindow will be forcefully shown on top.

In the test program, mainWindow will be forcefully shown on top when open QFileDialog, regardless of whether the parent object is mainWindow or nullptr. 

However, in qBittorrent, it is affected by the presence of a parent object.

some videos of behavior

test program : https://imgur.com/a/qFpSQDH
qBittorrent : https://imgur.com/a/VFq2GKB

Similar to https://github.com/qbittorrent/qBittorrent/issues/19072