I haven't really found out under which conditions the bug occurs, it just happens in about 10% to 20% of the times I play.
And if it happens I have to close the document, just quitting the game and starting a new one doesn't solve the problem.
I only tested it on a non-cx with 3.1.
Edit: I tested it on nspire_emu with 3.2 and had the same bug.