UDF values are overwriting the previously saved UDF values in users timesheet.
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If you're using a TimeSheet row level User Defined Field (UDF) to track certain data in users timesheet, and using In/Out TimeSheet format, then a change in UDF value for any row will affect all previous days of this timesheet with this newly changed value and the previously saved value will be lost.
A timesheet row level UDF allows only one value in a row across the entire timesheet period. This is the reason why changing the UDF value in one location changes it elsewhere as well. The solution is to use a cell level UDF which allows one value for each time entry recorded in the system. In this way, the users can change this value as much as they like without impacting other already set values.