This has just happened to my booking. Seat map was available the last few times I've checked and now 4 days before departure, it's no longer available. But the return flight still pulls up the seat map.

There's also a disclaimer message for the flight without seat map saying "For operational reasons, Cathay Pacific may occasionally re-assign seats without prior notice"

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​​​​EDIT: It's on CX metal. It's CX256 on 03MAY in PEY