Zoho People > Timesheet > Create
When you try to change a time log from Billable to Non-Billable or Non-Billable to Billable it will not change.
To see bug:
Create a new timesheet that has time entered
Go to a specific item and click on the pencil:
Change the Non-Billable to Billable or Billable to Non-Billable:
Click on the green check box and it reverts back to non-billable:
This becomes a significant issue when users first enter a non-billable project and then later switch it to a billable project. They have to correct it here but are unable to.