Stacking Issue with SII (Sulfur) data

I seem to have gremlins pop up when I stack SII data these days.  The following are stacked SII  images of IC 405 - Flaming Star Nebula.  The top left is the normal Batch PreProcessing (BBP) version from PI, the top right is from DSS, the bottom right is from DSS using an older batch of flats, and finally the bottom left is from DSS without any flats. 

 

There are no other issues with the other filters, narrowband or RGB.  This issue seems to be limited to SII and just since December - My last NB image.  See link: IC 410 Post (http://astroquest1.blogspot.com/2019/12/ic-410-tadpole-nebula.html).  Prior to that was my NB image of the Pelican Nebula from 10/1 which there were no stacking problems.

The above experiments indicate the problem is with my flats since I changed none of the setting in PI but I am still not 100%.  The DSS version using the latest flats (top right) was also weird but not as weird as the PI version.  In any case the, DSS version without any flats seems to be the best. 

The above image shows two sets of flats, the left side are the Ha from the most recent on the top and an older version on the bottom while the right side shows SII from the most recent on the top and an older version on the bottom.

Any thoughts or explanations are welcome.  The less technical the better. - Cheers