NSTimer not firing in modal panel

I have a repeating timer installed like this:

_cmdTimer = [NSTimer timerWithTimeInterval: 0.5
        target: self
        selector: @selector(timedTask:)
        userInfo: nil
        repeats: YES];
[NSRunLoop.mainRunLoop addTimer: _cmdTimer
                    forMode: NSModalPanelRunLoopMode];
[NSRunLoop.mainRunLoop addTimer: _cmdTimer
                    forMode: NSDefaultRunLoopMode];

The first time the timer fires, it opens a modal dialog. But then the timer does not fire again until the dialog is closed. I don't get that, since I scheduled the timer in NSModalPanelRunLoopMode. To verify that the dialog was running in that mode, just before opening the dialog I said

[self performSelector: @selector(testMe)
        withObject: nil
        afterDelay: 0.7
        inModes: @[NSModalPanelRunLoopMode] ];

and the testMe method did get executed while the dialog was open.