At AOA the CM even brought over the manager (I was pretty accepting of the answer but they went above and beyond to try and address my concerns!) and he said in no uncertain terms that the card messes up the system and he couldn't guarantee that after issuing the card to me that the other MB's in our party would work properly. Once again (just last month) I asked at BWI and the CM just politely advised me against this saying it causes more problems than it solves, and this time I was solo!
I'm guessing they do this to head off complaints with the door readers not recognizing the new ID versus the MagicBands they already set up for you, and to then not have to go out and finagle with the lock.
Unlike the networked tapstiles and FP+ entrances, the room locks are 'semi autonomous' in that they are sent a list of valid IDs and the date range they're valid for (probably for the next 3-5 days). Since they're battery operated, the locks probably only update their list of IDs (cards or bands) daily. If they handed you a card at the desk that's "new," it probably won't work for a day, and then they'd have to deal with aggravated guests complaining the door doesn't work with the card.
That's why if you get a new MB at the park, your FP+s work immediately with it, but the new MB tends to not open the hotel room until the next day or so.
I'm guessing the only way to get the door to update its list is to physically go to the door and trigger it to re-poll; that is, there may not be a way to "push" to the doors on demand. In cases of an eviction this works fine, since presumably the hotel manager will be there in person to trigger the re-poll.
Regular hotels don't have this problem because the locks are much stupider there - at a regular hotel, the next guest's card is what turns off your card. But at WDW, guests may have their room card weeks before they arrive, so the lock has to have a list of valid IDs and dates.
but also sad
