Strange Boxes Around Links Only On Mobile

Hello! I'm having this weird problem where all my links are getting boxes around them. What is stranger is that this only happens on tablets (not desktop or phones). It looks like this:

My website is https://bibliosmile.com/ and here's the code I'm currently using:

{border-bottom:3px solid;
border-image-slice: 1;
border-image-source: linear-gradient(to right, #a725f0, #fd1da4);
color:#000000}

Any ideas why this might be happening or how to fix it? Thanks so much in advance!