I have found this out several months ago - on top of that the activities sometimes would disappear completely. The accounting trail in the account activity details is VERY POOR to say the least.
What the defense we as members can have is to KEEP TRACK of the Account Balance each time you have done anything related to reward nights - be it redemption or cancellation.

Then of course we also need to keep track of the nights posted with the corresponding points. But that part is easier to spot error than the reward cancellation / point return.

With the way how Marriott handles the account activity entries, it really is upon us to keep accurate records of what we have done. There is no other way to go about it even though it should not be this hard.