I had my transmission rebuilt. It shifts rough going from 1st to 2nd. All other gets shift fine. What is it?

I spent 4.5k getting my transmission rebuilt. There's a slight clunk going from 1st to 2nd, but all other gears it shifts fine.

The car is still very driveable, but I'm worried this is just going to get worse in time / it sucks knowing I spent 4.5k for a clunk.

What do you think is causing it? I know they put in a rebuilt torque converter too.