I haven't worked in hotel operations in some time, so I'll defer to anyone with more recent working knowledge. In absence of that, here's my understanding of the issue.

Door locks are dumb. Each lock has a sequence of codes that will open the door. When a key is inserted (or sent via RFID), it looks to see if the key matches the current code and whether the "valid through" date/time has been reached. If both of those checks fail, the lock will look to see if the key matches one of the next codes in the sequence.

If guests never departed early, switched rooms, or lost keys, this problem would largely disappear, as the lock would simply fail to open until that "valid through" date/time was reached. As that's not reality, the only option is for the lock to open as long as the next code in the sequence is correct.

There are going to be times when someone makes a mistake. A Front Desk employee might a mistake. Or, perhaps a guest tells the Front Desk she is checking out a day early but decides to stay. If her keys still works, she might think that she doesn't need to tell anyone that she's staying. There are plenty of other scenarios.

The only way I know to completely stop the problem would be to connect every door lock to the key system. One option would be wi-fi. Obviously, that's a horrible idea because it's prone to hacking. The second option would be to hard-wire every door lock. For some hotels, doing that would require miles of wiring. It would be a very big expense to solve the problem.

Without question, report this every time it happens. It's possible that someone working at the hotel is skipping a step or hasn't been properly trained. But, sometimes hotels can do everything perfectly and the problem will still happen. Hotels know this is a problem...it's just not big enough to warrant spending large sums of cash to fix.

My advice: Always engage the deadbolt.