SB 5000 + Z9 auto ISO question

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Just received my SB 5000 and though its not life threatening, when my auto ISO is on and the setting calls for 6400, when the flash is attached the ISO drops to 400. When the shot is taken the exposure from the flash is correct. When the ISO is set manually and the flash is attached, nothing changes with corresponding shot being accurate as well. The only real problem is the viewfinder is dark when the ISO changes to 400. I have read through the manual. I have checked my Z9 settings including Maximum ISO setting for flash. I am missing something and haven't found it.