The reason this "feature" was passed through is likely because:
1. It was not a use case defined in the requirements (ie, an oversight in the requirements)
2. The engineer coding the FP system didn't talk to the one coding the reservation system.
From their point of view, both systems were worked as designed - however once integrated together issues like this bubble up. The systems were not designed to wipe fastpasses - however also were NOT designed to migrate fastpasses when new reservations for the same
MDE during the same time frame were created. What if they just wanted to book a second room...duplicate all the fastpasses?
The short term solution appears to be for the CM to advise folks that altering their reservation will wipe out their FP+ info (why ADRs are OK is beyond me).
Long term, new requirements will have to be drafted and incorporated into the software in the future, either as a critical patch or flowed into their software roadmap.
This does not make it OK, and everybody who reads this should be aware that changing reservations can wipe out fastpasses, so caveat emptor