If user merges some column labels e.g. Comments and import mode is set as Start New Columns. Then Origin will treat the merged column as a unit and check if 1st column in the unit is blank or not. If blank, import into it. If not, start new column after the unit and import there.
System vairable @DCM=1;//disable check merged labels. Then Origin will work the old way before 2018b. Import into empty columns in the unit.
Columns with missing values are still treated as columns with data. In the past, when user tried to import multiple data into same sheet with Multi-File Import mode set to Start New Columns, data will be imported into new columns after missing values columns. In 2018b, data will be imported into those missing value columns.