99.9% of people having this problem have also forgotten promotions or genius discounts on their OTAs such as Booking.com, Expedia or AirBnB which have independent minimum stay restrictions (non-connected with OTA Sync).
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Check if you currently have certain promotions set on the sales channel, if a special package is active for you, or if the reservation was entered before you made the minimum stay change.
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Also you should check under which rateplan reservation was booked and check-in Setting > Pricing Plans > Specific Pricing Plan Settings which Restriction plan is mapped under it and then check for that Restriction plan the minimum number of nights for booked period.

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