XPS13 9370 odd switch on behaviour

Hadn't used my XPS13 for a couple of weeks and went to switch it on today. Pressed power button and nothing happened, tried several times - both short and long presses to no avail; pressed the power monitor button and no LEDs showed so assumed I'd inadvertently left it switched on and the battery had run down.

I then plugged in the mains adaptor & it switch on fine. However the battery charge icon showed 53% power remaining immediately suggesting a dead battery was not the problem. When I pressed the power monitor button, it now showed LEDs lit too. I think I may have had another occasion when it didn't switch on first time but did the 2nd time ... I'd dismissed that as 'user error' but not so sure now.

Why would it not switch on until mains lead plugged in and then show 53% power remaining? Any thoughts folks?