The edits endpoint can only modify the image in areas where there is a 32-bit alpha layer that is completely transparent.
Here’s a 256px PNG that has the middle 1/3 as transparent. Click “download” or right-click and save and hopefully you’ll get a version that the forum hasn’t re-encoded, and then use that as your mask file to see if the transparent area is re-drawn.
Edits won’t be able to redo the whole image as a watercolor (and variations takes no prompt).