Yeah, this one's common. The NV356E canister is designed to be lightweight and detachable, but it doesn't have a built-in handle or a super-wide base. It's top-heavy, especially with a full dust cup or an attachment plugged in.
The fix is about placement, not a repair. When you're using it in lift-away mode, set the canister on a flat, level floor. Don't put it on a rug or an uneven surface. Also, try to keep the hose as straight as possible behind you—sharp bends or pulls can tug the canister off-balance.
IF THAT DIDN'T WORK: Make sure you've fully clicked the canister into its dock on the upright handle when it's *not* in use. If the docking latch is worn or not engaging, the canister won't be secure even when attached. Check for any visible cracks or damage on the docking points.
HEADS UP: It's just a design quirk of this model. For above-floor cleaning, it's often easier to just use the whole upright unit and tilt it back rather than detaching the canister for every single task.