When the battery dropped to 30% power I plugged it into the charger and continued to use the phone. I expected the phone to recharge but it continued to drop down to 28% and then remained there. On another occasion the phone recharged to 84% and wouldn't charge anymore above that.
This only happens when the phone remains switched on. If you recharge with it switched off it'll go up-to 100% charge.
Through trial and error I've discovered a workaround which is switch the phone off and plug it into the charger. When it starts charging switch it on. Now you can use the phone and it'll continue to recharge up-to 100%.