I am not sure if reference manual mentions it somewhere or not but values 0x0000 and 0xFFFF cannot be used for Censorship Control Word (0x00FF_FDE0) or Serial Boot Control (0x00FF_FDE2).
Although I have found it described anywhere is the device’s RM, some related MCUs (for instance MPC5604B) has this restriction aslo specified for passwords:
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When configuring the private password, each half word (16-bit) must contain at least one "1" and one "0".
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I suppose it works same way for all MPC5xxx devices.