The output PDF file from the Print widget of Web AppBuilder for ArcGIS does not maintain the scale specified in the Force scale option when using the print service.
The templates that came with the default print service in ArcGIS GIS Server 10.6 and 10.6.1 computed both the size of the scale bar and the scale text based on the center of the extent being exported. This was in line with Map Viewer in the ArcGIS Enterprise portal and ArcGIS Online but differed from the behavior of Web AppBuilder for ArcGIS and the expectations of many users.
Beginning in ArcGIS GIS Server 10.7, the scale text is now computed at the equator rather than the center of the current extent to match the expected behavior of many users and to match the scale text shown by Web AppBuilder for ArcGIS.