Have you tried tapping the account icon in the top right hand corner of the fitbit mobile app, then scrolling down to advanced settings. You should be able to manually set your time zone from there. Toggle the automatic time zone to off, then below that select the timezone you are now in. Hopefully that solves your problem.
Also if you haven't already done so, it might be worth restarting your tracker (press and hold the left button and bottom right button together for 15sec).
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