This can happen if a player activates another Item of the same effect at the time when "Offense and Defense +50% (7 days)" is already active.
For example, a player activates the "Offense and Defense +50% (7 days)" Item and then activates the "Offense Enhancer: +10% (4h.)" Item. The "Offense Enhancer: +10% (4h.)" Item replaces the first one since such items cannot be active simultaneously.
Please keep in mind that there is a pop-up notification that warns players that the "Offense and Defense +50% (7 days)" Item will be deactivated once they activate a second Item, and it always appears when they try to do so.
Here is a screenshot of the pop-up notification from the game: