The bicep has three main functions: arm flexion (the curl), forearm supination (which is why I prefer a straight bar) and shoulder flexion (rotating the shoulder up, lifting the arm up). So finishing the curl with a shoulder flexion makes sense.
But in the past it always felt like I was losing the contraction when I did that. Why? Because when I did the shoulder flexion I'd let the shoulders go up and would let the traps kick in. You want the shoulders to rotate up, not to be lifted up. So you must work extra hard to keep the shoulder blades retracted and down.
Do not let the traps fire up. If you don't feel a stronger biceps contraction you aren't doing it right and need to practice.