Since the fitbit gets the date/time from the phone, try changing your phones date time to the date of interest. Then try changing the start date of the weight.
To set your phone date/time (Android), I would try:
System
Date & Time
turn "off" network time and try changing the date (time too if you want).
When you open the fitbit app, on your phone, click your picture (top left),
then click "Activity & Wellness",
click "Nutrition & Weight",
click "Your Goal" and cycle through the options and it takes the date you set the information.
See if those steps help. Be sure to set the phone back "on" network time.
