There may be a bug in the RHI D3D12 backend.
I wrote a test program that starts a texture readback in each frame.
Here is what happens with different Qt versions and different RHI backends:

Qt 6.8.0, 6.8.2:
    OpenGL: works
    D3D11:  works
    Vulkan: crashes in QRhiVulkan::finishActiveReadbacks
    D3D12:  crashes in QRhiD3D12::finishActiveReadbacks

Qt 6.9.0 Beta 2:
    OpenGL: works
    D3D11:  works
    Vulkan: works
    D3D12:  crashes in QRhiD3D12::finishActiveReadbacks
    
Here is the output I get in the console window with Qt 6.9.0 Beta 2 and different backends:

OpenGL, D3D11:
    Readback 0 started
    Readback 0 complete
    Readback 1 started
    Readback 1 complete
    Readback 2 started
    Readback 2 complete
    Readback 3 started
    Readback 3 complete
    Readback 4 started
    Readback 4 complete
    ....

Vulkan:
    Readback 0 started
    Readback 1 started
    Readback 0 complete
    Readback 2 started
    Readback 1 complete
    Readback 3 started
    Readback 2 complete
    Readback 4 started
    Readback 3 complete
    ...

D3D12:
    Readback 0 started
    Readback 1 started
    Readback 0 complete
    Readback 2 started
    Readback 3 started
    C:\Repos\ReadBackIssue\x64\Release\ReadBackIssue.exe (process 5796) exited with code -1073741819 (0xc0000005).

I am using Windows 11 and Visual Studio 2022.
The test code can be downloaded from https://github.com/jrade/ReadBackIssue.
The backend can be changed in Main.cpp.