I'd be bothered by it too. Recently, when he was checking to see if we could meet at the usual time the following week, my marriage counselor said he'd have to have a "hard stop" at 2:45 (our appointment is at 2, and they're 45-50 minutes). I asked for us to schedule a different time (and we did). I didn't tell him this, but it's because he tends to run late, so I figured the chance we'd get our full session in would be slim. Turns out it was good we rescheduled, because we had a pretty intense session, and he kept us a bit over, and the extra time helped us resolve something. So glad we didn't have a hard stop. Keeping clients over at times is one reason he runs late, but it's often been helpful for us, so I am OK with him doing the same for other clients, as long as it's not like 20 minutes late every time.

I'd definitely say something to your T though about it bothering you. Hope you get to have most of your session today!