I had the same problem. It seems that the LESSON module doesn't like to be changed in the middle of theactio, that is, after students take the lesson, if we change sommething its dificult for the activity to go smooth.
Another it apears to me, it was a student getting a 100 % in the grade but I didn't validate the last five questions, so only after I assess the last question should he have the 100 %. I have to investigate this.
Bye António