What is the default sort order in the Board views of the Modern UX?
The sorting is done by "position" value.
A sample REST Call that is made while fetching a Board view is as below. Bolded text is the relevant part about the sorting question.
http://some_server/ppm/rest/v1/private/views/5124831?expand=(listColumns=(limit=500,sort=(position+asc),fields=(name,position,isSelected,metadataURL,pinned,sortOrder,sortPosition,width,groupType,isGrouped,layout)),viewSettings=(limit=200,fields=(layout,name,value,classifier)),viewTargets=(limit=500,fields=(attribute,position)))
The position value is determined from the odf_ui_list_cols table. You can determine which columns are in which position by running a query like the one below. The internal ID of the view will have to be passed. The internal ID is available from the REST Call.
SELECT
*
FROM
odf_ui_list_cols
WHERE
view_id = 5124831
ORDER BY position ASC;