If either of these answers don't solve your issue please let us know on customer support and we will investigate further.
1. Check your spot MLOS for the date/dates in question
Use the "hidden rows" button on the calendar to show the spot (ie. daily) MLOS (Min Length of Stay).
Spot MLOS overrides the MLOS restriction set on the property settings.
Spot MLOS should be reflected on all connected booking sites
2. Check your rate plans on Booking.com
Additional Booking.com rate plans (eg. non-refundable rate plan) does not share MLOS with the Standard Rate plan Uplisting is linked to, by default. You can ask Booking.com to change this (details below).
The most common reason for bookings less than the min stay (MLOS) set is due to additional rate plans on Booking.com.
How to check your rate plans
First, go to the Rate plans page on your Booking.com extranet
If you have multiple rate plans, the booking with a lower MLOS than expected is most likely not-booked on the Standard Rate.
Although prices are linked to Uplisting on all rate plans (if setup correctly), restrictions are not by default.
How to link restrictions to all rate plans
Contact your account manager or Booking.com partner support and ask them to attach all rate plans as a child rate to the Standard Rate (ie. the rate plan showing as mapped to Uplisting). The child rate plan will then follow restrictions of the parent Standard Rate.