If you are truly stuck at 9970 Light it is a bug. Once you get to 10,000 light further light choices will begin decreasing your dark points until they reach 0. In any event, even if you do stay at net 9970 light, that is Light IV, not gray or neutral.
If you try another light side choice and you don't move closer to net 10000 Light, then it is a bug. Remember that Light side choices during a quest don't actually register until you finish the quest. This was working fine for me.