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.