I am pretty sure that's because they messed up with the bar itself. It doesn't display your gauge but the whole group's gauge and if you're seeing it drop rapidly then that's because someone is twiddling their thumbs and not pressing anything, if everyone is mashing atleast one button that bar should be staying between 100 and 90% the whole 10 seconds. This is also why you can fail the QTE even though the bar is at like 50%, because 50% means someone didn't press anything and technically failed the thing.