Clicking on (or doing a keyboard shortcut) 'loop playback' stops playback

If I'm playing a sequence, and I've realised I do/do not want it to loop at the end, if I click the 'loop playback' button (or press a keyboard shortcut that I've allocated to it) playback stops. It shouldn't; playback should continue until the end of the sequence, then PPro should take notice of the new state of 'loop' and then decide whether or not to stop playback or loop it.