I can't set work completed to 100%. Gets reset to random value !

I'm using version 1.5.9. I have a task that I want to update and adjust the work completed percentage. When I select the task and click on update I get the update box ok, but when I change percent completed to 100% and then close the box, the value is auto adjusted to a random value%

If I then reopne the box and try again, I get a different auto adjusted value : 77%.