No it’s not normal under normal conditions.
Air compresses and expands in cold and hot weather. It could be hot during the day, then overnight the temp could drop substantially, compressing the air in the lines and causing the rear to sag a bit. Then in the morning the system will level the rear. This is just how your tire pressure can fluctuate in the same conditions.
Other than weather playing a part, if the temps are not drastic like the above example, the system should hold the air with no issues.
You should get your hoses and connection points in the system checked for leaks.
