had the same issue a few months ago - 4 months worth of data just disappeared! i reported to support and just got useless responses from them claiming i was logging in with the wrong account (even though i still had data showing before and after the missing period) and it was all ok prior. they then sent me an e-mail address with was apparently my correct login - it wasn't, it was someone else email address!!! not sure they are aware of GDPR!
sent the support print screens of my day/week/month/year view to prove it, and the only response i got was, it's not showing because you are in the wrong view! i was trying to show them how it was missing from those period becasue the graphs were showing nonsense!
after about 3 weeks of no response the data magically re-appeared! no idea how/why - not much help sorry, the support from fitbit has taken a massive nosedive in the last 6 months - no idea why, it used to be excellent!
