Welcome to YT!
I have 4 yorkies. 3 of them has did that too. The only thing that has ever worked for me is those tablets you can buy called dis-taste. They make their poop taste bad so they stop eating it,i kinda thought it would taste bad anyway
Sometimes they will outgrow it and sometimes it just gets to be a habit. I think mine just got in the habit of doing it. When i gave them the tablets, they stopped. After i ran out of tablets, it wasnt long they started doing it again...i just got some more of those tablets today. Good Luck!