Roblox Accessories with WeldConstraints have problems. This fixes the shifting that happens when Accessories with welded parts are parented to the Character.

addAccessoryWithWeldConstraints.ts

function addAccessoryWithWeldConstraints(humanoid: Humanoid, accessory: Accessory) {
	const handle = accessory.FindFirstChild('Handle') as BasePart;
	if (!handle) {
		return;
	}
	humanoid.AddAccessory(accessory);
	const os = handle.FindFirstChild('OriginalSize') as Vector3Value;
	if (os) {
		const sizeChange = handle.Size.div(os.Value);
		handle.GetDescendants().forEach(desc => {
			if (desc.IsA('WeldConstraint')) {
				const { Part0, Part1 } = desc;
				if (Part0 && Part1) {
					const weldCF = Part1.CFrame.ToObjectSpace(Part0.CFrame);
					const orientation = weldCF.sub(weldCF.Position);	
					const weld = new Instance('Weld', desc.Parent);
					weld.Part0 = Part0;
					weld.Part1 = Part1;
					weld.C1 = new CFrame(weldCF.Position.mul(sizeChange)).mul(orientation);
					
					desc.Destroy();
				}
			}
		});
	}
}

Roblox Dev forum post about it is here

Conversion to Lua is here