Confirmed to me today by the Department of Transport (Belize), buses are currently not allowed to cross at any time on weekends.

This is because the scanner is not in operation on weekends, but it could change in the future so that buses can cross only between 9 am to 5 pm on Saturdays & Sundays.

This only applies to large vehicles (regardless of whether they are Belizean, Guatemalan, Mexican or anything else), so taxis and small vans can cross.