It is a product catalog site.
never had that problem. Can you make sure this behaviour is not caused by any other styling on the website? For example another content editor webpart with js/css inside or the master page and it's related css could cause such behaviour.
Did you have a look at your browsers developer tools? Any useful information there?
I checked all of that and did not see anything. I changed the border and the background color as a workaround.