The description of a timesheet line is also saved on the other selected record
When changing the description of a timesheet line, you have to de-select the field to make the save-button available (it's standard greyd out). So you either select a field on the same line, or you select another line. In the last case, when you select the description field of that line, the changed description is also stored on that line, so you loose that description.
To reproduce the issue:
1. create 2 timesheet lines. It doesn't matter if you do that on the complete list with all the timesheet lines or via the wizard for filtering per date.
2. select one timesheet line and change the description (you stay in list-view). Mention that the save-button is greyd-out
3. select the description-field on the other line
4. click on the save-button. Mention that the description is also stored on that line
When you do the above again but select another field on the other line, it works like expected.