MessageBoxEx sometimes shows as hollow window, border only, and only on Windows 11

Question

At places in my app where I call MessageBoxEx() to display a question or information, only the dialog's border and its title are shown. No message and no buttons are visible. And the dialog is transparent, so it shows whatever is behind it.

I can hit that place in the app ten times and nine times it displays just fine, but once in a while the box is hollow, and if I press Esc and then again invoke the command that shows a message box, it will display correctly.

My app is almost 20 years old (well, most of the code is, I make changes all the time), and I have never seen this before, just this one fresh Windows 11 install.

Is it me, or maybe I should ask my client to wait until the next update maybe?

UINT  msgStyle = MB_TASKMODAL;

if (...) {
    msgStyle |= MB_ICONSTOP | MB_OK;
    titleText = "Stop";
}
else {
    msgStyle |= MB_ICONQUESTION | MB_OKCANCEL;
    titleText = "Pitanje";
}

itemHit = MessageBoxEx (NULL, errMsg, titleText, msgStyle, MAKELANGID(LANG_CROATIAN, SUBLANG_DEFAULT));

(the word Pitanje means Question in Croatian)