How do I get rid of the boxed rule around highlighting

Hello! I have been using MS Word for years and a new problem has just popped
up that I can't undo.

When I highlight text, a box appears around the highlighted text when I
print. It's a thin rule that does not appear on the screen view.

I think this happened about a year ago and I must have finally figured out
how to turn it off, but now I can't remember and I've tried a bunch of stuff.

Help!

(Thank you!)