Doesn't have to be, but it is a common trait amongst many single handers (as you have noticed). It isn't something I would necessarily teach outright, but if, as a function of everything else being right it happened, then I wouldn't be worried. It is most likely because the player is pre-setting wrist extension.
I would only "correct" it if it was effecting the contact in some way, for example they were open face at contact.
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