Bug for image in lightbox??

Don't know if this is a bug, but if I put in my article a thumbnail image opening in lightbox, a blue line is all around the image after closing the lightbox modal window (see attachment) and I have to load the page again to make it disappear; the issue is both in Chrome and Safari.
Working in localhost, Joomla 3.8.5, Gantry 5 framework, Inspire Theme template "Solo"
Thanks for your help!