Right click the table and select "Script table as" > "Create to" > "New query editor window".

Do you see the default constraint script there?

Please post results of the following query

SELECTdc.nameAS ConstraintName

, dc.definitionAS ConstraintDefinition

, dc.type_descAS ConstraintType

, c.nameAS ColumnName

FROM sys.default_constraints dc INNER JOIN sys.columns c

ON dc.parent_object_id = c.object_id AND dc.parent_column_id = c.column_id

WHERE OBJECT_NAME(c.object_id) = 'yourtablename'