Yeah I can see your issue now. I use Comsec App on mobile, and the browser on a laptop (and yes I trialled the new watch list on the laptop), so I hadn't seen your issue.
As you have described, trialling the new watchlist when viewing the website via a browser on a mobile in portrait mode seemingly limits it to a single column. You then have to rotate the device to see other columns. I've tested it out and can see your issue.
Unfortunately I don't think many people would use the website that way? The site will have been optimised for desktop and the app would have been optimised for mobile device.