well youve already tried the one thing i was thinking to recommend, changing to jpg mode.
let's see if we can try an experiment:
capture a window once when screenshot captor is NOT runing, and just do a ctrl+v when in windows file explorer to save the image data to a file. Test paste in mail make sure it works.
then capture same window when screenshot captor IS running, and do a ctrl+v again in windows file explorer to save this image data to a file. Test paste in mail make sure it still doesnt work, as expected.
Send me the files (firstname.lastname@example.org
Maybe you could also send me a screenshot of how it looks when you paste it into the mail program.
The one thing i can think of is that maybe sc is creating a high-color image and the other isnt, and somehow thats confusing it..