U3014 power on/off button issue

U3014 (purchased 2012) replaced under extended warranty by reconditioned monitor. "New" monitor exhibits strange behaviour with power-on switch as follows:

Set-up: new MacBook Pro, 13" retina display, connected to U3014 via Mini Display Port input

1) With connector in monitor, but disconnected at laptop end.  Power up laptop, power up monitor - monitor on-/off switch and selection buttons glow as it searches for sources, so ok so far.

3) Plug in laptop mini-DP - successful connection between the devices and monitor springs into life.

4) Switch monitor off at the on/off button - monitor goes off and the on/off button is no longer illuminated.

5) Press on/off button to get monitor to display again and nothing happens and it appears as if the monitor is dead.  It doesn't power up at all. To get the monitor to work again it is necessary to unplug the mini-DP cable and then press the monitor on/off button - after which the monitor is ok.

This behaviour is the same when using the full-size Display Port connector, but no such problem when using the HDMI connection.

Problem did not exhibit with the original monitor.