There's nothing wrong with that happening at all, it would be infinitely worse to force your hands to stay rigid and introduce a whole load of tension.

The principle motion should come from the wrist but some sympathetic movement in your fingers or arm isn't automatically bad.
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Or you could attempt to pick in the manner of my guitar teacher, which is all fingers (I think its sometimes known as circular picking), and movement of wrist and arm for navigation around the strings.

I use a combination. Some movement of fingers, and wrist. Keeps tension low in my hands, and makes me quicker.

Or you could pick like Al Di Meola. All forearm, rigid wrist and hand.

The point is that there's not just one way to pick.

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