Until you press the stop button. Then the momentum in the spinning workpiece can loosen it. I always keep the tailstock up as much as I can.
I've dealt with an out of balance blank, too thick for jig saw and before I got a Bandsaw, like this. Get as good as you can with bowsaw or chainsaw, drill a shallow hole in the centre at the mounting end (in your case the midpoint of the faceplate), put a rod or screwdriver sticking up in a vice, balance blank on your little hole and use chalk to mark the heavy point. Use a small axe or whatever you have to hack some wood off there. Repeat, repeat.... Hard work but eventually get something enough in balance to be able to spin at low speed without the lathe wandering across the floor. But only if you really really need to and have plenty of time and energy.