Dialog DPI scaling issue with multiple monitors under wayland

My setup is:

• Debian 12 "Bookworm"
• Window manager is GNOME with Wayland
• Multiple monitors with different DPI scaling setup in GNOME:  Monitor 0 has scale 250%, 1 (primary) has scale 100%, 2 has scale 100%.  The problem still happens if I use 200% integer scaling instead of fractional.

Problem description:

• I've attached a minimal reproducer
• If my Qt application opens a modal QDialog when on monitor 1 or 2 then the dialog appears incorrectly: its window decorations appear incorrectly scaled. 
• If I resize the dialog or drag it to another monitor then it instantly changes to look correct and stays correct no matter how it is moved or resized.
• If you drag the parent window to monitor 0 and then the dialog always appears correctly.
• If you start the parent window on monitor 2, then drag it to monitor 1, then the first time you open a dialog it appears correctly, but the next time you open it it looks incorrect.

Debugging/hypothesis

• The problem goes away if I use XWayland
• The problem goes away if I set all monitors to 100% scaling
• The problem goes away if I only use a single 250%-scaled monitor
• I have also reproduced this with Qt6 (6.4.2)
• I think what's happening is when the QDialog is initially created it thinks it's on monitor 0 so assumes 250% scaling.  When it is actually shown on a different monitor it doesn't seem to update the scale.  If I drag the window to another monitor or resize it then it suddenly notices it's not on monitor 0 and updates its scaling.

Other notes

• When the QDialog is empty the problem appears as the window decorations being incorrectly scaled.  If there are other layouts/widgets in the dialog then the decorations appear correct but the dialog's background is rendered incorrectly, apparently showing random uninitialised surfaces.
• In the screenshot, the mainwindow (the one with the button) appears with correct scaling but the dialog (to the lower right) has decorations which are 2.5x larger than they should be