Beyond Frustrated ..... Why does adding underlay changes the top fill ????

I am so frustrated. Something simple should be easy to do.

I import (or open) a DST file of a monogram. This is font with special fill created in PersonalizeNStitch and saved as DST.
I select objects that need underlay
I add underlay
Adding underlay changes the special fill stitch of design (this happens with both the column stitch & tatami fill objects with both the DST and PES version of this file)



AHHHHH

I can digitize object to create my own underlay, but WHY do I have so many problems?
Wilcom is great for digitizing; but editing existing sewing format files is really proving to be a challenge for me! This is so frustrating!

That is normal, and happens on any software, try buy files from people can offer the EMB version too.

Thank you Sam, Alex and Luiz for the explanation.

I'm disappointed there is no magic wand to fix this, but the explanation makes sense. It also answers another problem I've had (why resizing a non-emb design can affect stitch generation/patterns).