I want make reservation for one day at LM Malta I have this answer about Platinium Guaranteed room availability form SPG chat :

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Valerie H.: Platinum overrides are not in place for one night if the hotel has imposed a minimum length of stay, sir. Please note, that hotels may decide to impose a minimum length of stay at any time based on factors such as room availability, local events, etc. Please also note, as rooms that were designated to third-party websites are not included anymore in the hotel's inventory, a minimum length of stay policy would apply to rooms booked from us. We have no control over the availability information on third-party travel sites, I am afraid.
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And this date is aviabke on hotels.com, booking.com etc. and this is not blacklisted date.