Hello, please follow the 3 minutes steps below to lost data and verify the bug.
Version: 7.6.4.1 (X86_64) / LibreOffice Community
Build ID: e19e193f88cd6c0525a17fb7a176ed8e6a3e2aa1
CPU threads: 8; OS: Windows 10.0 Build 22631; UI render: Skia/Raster; VCL: win
Locale: en-CA (en_CA); UI: en-GB
Calc: threaded
1 New Spreadsheet, input [0,a],[0,a]
2 Save as Text csv(.csv)
3 in clipboard prepare [b],[b],[b] then select column B to paste it
4 Select A1:A3 then press Ctrl+D for fill down

5 Save, then File\Reload

[0,b],[0,b]
DATA LOST!
It is unavailiable for .ods format in my test. After you verify it do me a favor that you send the bug report rather than me.