GtkFileDialog becomes modal for the original window group, not for the parent's

Steps to reproduce

You can even reproduce it in gtk4-demo!

1. Open gtk4-demo
2. Open the inspector (which is in a separate window group)
3. Go to the CSS page, click Save
4. Observe how the inspector window still accepts input, but the gtk4-demo itself (which is in the original window group) doesn't

Current behavior

The file dialog becomes modal for the original window group.

Expected outcome

The file dialog becomes modal for the parent window's window group.

Version information

F40 and org.gnome.Sdk//master both have this issue. Wayland, niri.

Additional information

I hit this in Identity where every window, including the first one, happens to be in a separate window group.