Mine does this, too. He's been known to bring them back up to 30 minutes early. He sends them up to the door & they bang on the door & ring the doorbell without ceasing until I open it. When I'm NOT home he has them call me on my cell & demand to know where I am and when I'm coming home.
He tells the kids that he's not allowed to be even one minute late, and if he is that one minute late I'll get him in trouble. He scares the kids with this, and then I'm stuck trying to educate the children about visitation & time schedules and convince them I'm not mad, never was mad, just confused why they are home early and yadda yadda yadda
I tried addressing this with him, then via the lawyers. He doesn't care. All I did was waste money with the lawyer trying to convince him to just show up around the right time.
You're not alone.
Maybe you'll also get this particular trick: he brings them back early or on time, then sits out front & plays with the kids long past when his parenting time is supposed to end. Gets an extra 20 or 30 minutes that way.