Implements should not be left in the air when the tractor is shut off, as they absolutely WILL drop, and you're creating a potential safety hazard. So that is definitely normal.
I wouldn't expect much, if any, drop while the tractor is running. 1/2" in 1/2 hour wouldn't bother me, especially for a backhoe. I don't think there are any usage scenarios where 1/2" in 1/2 hour is going to affect using the tractor, are there.
Now, if the front loader drops in seconds or minutes while you're using it, then there is a problem. It should not drop in realtime, or in the period of a few minutes. If it does, then that suggests a valve is bad or there's a leak somewhere.