[Updated] v.159 - MapleStory X Attack on Titan Update Notes

The Master Craftsman's Cubes from the Daily Endeavor event don't expire. They also don't stack (i.e., one to an inventory slot), but they are tradeable in the account.
Worth farming even one doesn't have any use for them right now.

... And now they do expire.
Guess that was important enough to update in the Unscheduled Maintenance.