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.
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.
view_id = 5124831
ORDER BY position ASC;