Using sample code from mapwingis importshapefile I tried to use a an existing postgres/postgis layer instead of shapefile
In this case, if original layer has empty values, it appears that destination values are filled with previous column content (see joined file) top is original layer, bottom is imported layer.
I will try to reproduce.
Note that this looks like the issue that was fixed for MWGIS-128. I created a link.
Could you check if this problem still exists in the latest version of MapWinGIS?