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.
Don't be upset by the results you didn't get from the work you didn't do...
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