@kiwidude My preferences in Calibre are the same as yours: No SVG Cover and Preserve cover aspect ratio are unchecked. Indeed nothing is check in the menu.
Now the good news is checking Insert or Replace Cover has solved the border problem without doing a conversion in Calibre. I thought I had checked that box and so I am pleasant surprised it is working. I will try more tonight, but I have to do some yard work to keep up with neighbors!
If this works as I hope, this will be terriffic news.