If your aro is randomly losing scales, then no its not normal, but if it gets into fights or accidentally rubs up against a rough rock or something, then yes.
I would just watch your aro very carefully over the next few days, see if anything is up.
Mine started to lose some scales on one side of its body about a month or 2 ago for no apparent reason. At first I thought my oscar might have done it, but when I watched it for a bit, I realized that it likes to swim between this rock I have and the side of the tank. It's been doing it ever since it was little, but now that he's bigger, he doesn't fit in the hole as nicely anymore but still tries and every now and then, the rock would catch one of its scales. So I ended up moving the rock and his scales stopped falling off. Just depends on the situation.