I suspect the weight of your build platform is sufficient enough to turn the Z axis leadscrew against gravity and overcome the friction within it and the motor. I've never seen a printer do that myself, so I don't know if that's indicative of any kind of failure on your particular model. When the stepper motor is energized it holds itself in position which is why this doesn't happen, for example, during a print.

If I were you I would just move the bed via software or your printer's control panel if it presents the option. That should be safer, faster, and not slowly give you carpal tunnel as an added bonus. I haven't done the math via figuring out the screw thread pitch and so forth, but for example my X-Max 3 has a total Z axis travel of something like 330mm, so it would take an entire month of Sundays and hundreds of manual revolutions to screw the damn thing from bottom to top by hand. I have a machine precisely for doing that for me...

If you have only one Z screw you are fine. But if you have 2 Z rods with stepper motor on each one, they can get out of sync when losing power. You can print Z motor brake to solve that problem

It happens, and has happened to me while moving it manually, especially if I stop and let go abruptly. If I do it gently, it’ll stay in place. It also happened a few times when I didn’t screw the Z coupler properly so it just spun out. The weight of the print head along with the general resistance and stepper rotor magnetic resistance should be enough to make it stay still, and I assume it stays still as well when you turn off the printer. Or does it just crash onto the build plate?