If it was a fill from empty I would say yes mate, perfectly normal.
The air heats up because of the pressure and density, as it cools down it becomes less dense meaning there is less pressure in the tank.
I usually fill to 4k from empty, then go back after 2-3 hours and top back up to 4.5k and it's fine.
If you check it again in 24 hours and you've lost more then you have a leaky reg, but I would imagine you listened to the tank already?