fat mass, muscle mass, etc. that are deleted

Same issue here.

Got the same problem. Also it seems that you can see latest data about body fat and muscle mass in (at least) android APP dashboard, but they don't appear anywhere else. Got my device this Monday and thought it has broken already.

I deleted the garmin connect app, re-installed and reconnected the scale to the app. I connected the scale to an internet network that I can turn off.
I weigh myself.
Control in garmin connect and all metrics appear.
I turn off the internet at home and go out.
(The scale is not operating and is not connected to the internet, the modem is switched off)
Lunch time
The data is still present

now I've checked and it's all canceled.

The connect is not connected to myfitnesspall and Apple's health app.

I had this exact issue with my iphone6s. I then disconnected the link to My Fitness Pal. I then went on to the Health app/Sources/Connect and turned all categories off. Whilst the history remained gone, the new weight measurements are ok, with all the details. Seems like there is a bug when linking other apps on IOS.

i've lost all body composition data from Index scale excluding weight since January 10th. I've had a connected My fitness Pal app since last September but not been adding calorie data to My fitness pal until January 10th! now disconnected my fitness pal app from GC and will see how that goes.

I manually enter my weight (and I don't do it every day) and I noticed that it automatically updates my daily weight to the number in user settings in the garmin connect app.

Look at my video
Garmin fenix 5: 7.91 and 7.60
Balance: Index scale 3.0.0 https://youtu.be/AodW35ZNCjQ