I noticed that when I start going heavier on the barbell rows, that the motion starts turning into the romanian deadlift after the first rep or so. This involves unfolding of pivotted hip into a straight line on the concentric. Are the rows supposed to be done at a lower weight range to avoid this, or is this some general form mistake from my part?
It's too heavy for a strict row if you have to hip hinge.
If you find yourself using a hip hinge to help you get the weight up in a row, then it means you have too much weight on the bar. Your body is compensating for your insufficient lat strength by engaging the hip extensors. If you want to stop, you need to lower the weight.
Now, this isn't "wrong", it's just different. Personally, I don't do rows for hypertrophy or rowing strength, I do it as an accessory for my deadlift, so heavier rows with a bit of hip hinge may even be more specific to the deadlift than "strict" rows.