In Classic Timesheets you see more Other Work tasks to add to a timesheet, versus the Modern User Experience (MUX) where some Other Work tasks are missing / not showing.
Release : All
The Other Work items that are not available to be added to the timesheet in the New UX need to be opened for time entry either by going to the task list in the UI with a URL similar to the following or by using XOG.
Where the nnnnnnn at the end of the URL is the Other Work's internal ID.
The Classic UX does not validate whether the effort task is open for time entry, just whether the Other Work itself is. The MUX on the other hand does validate this, which explains why some of the Other Work items are not available to add to the MUX timesheet.