Modal Window seems to interfere contents in another Window on macOS 15

I would say it is a follow-up of QTBUG-135058. The reproducer is similar but with 2 changes:
1. Upon Timer times up, a Rectangle is also shown in the original window.
2. The separate window is modal.

Steps to reproduce:
1. Build the reproducer against either 6.8.3 or 6.9.1 on macOS 15. Notice that the problem is better to be tested on macOS 15 since macOS 14 may not even allow you to proceed to the following steps.
2. Click the button and then minimize the window.
3. Wait until a separate window shows up.
4. Restore the original window. macOS 14 may fail at this step. But macOS 15 allows it.

Expected: a red Rectangle is shown at top-left corner the original window.
Actual: nothing is shown. Not even after you close the separate modal window.