JS Code that directs away from my sites injected by theme/sample templates

Hi.

I installed the GeneratePress theme on 5 sites, chose, applied and edited a different template from the site library for each, and unfortunately, all of them have suffered the same issues; after a while, opening any page/post on the sites, including the home page, redirects to some different sites.
Luckily, I'm a bit savvy and I found 2 vectors of attack, one was some "jquery-min.js" and a js script with some weird src injected into the body of all the pages/posts that didn't appear in the editor or in the database.

This problem disappears immediately when I change the active theme to one the themes that come with Wordpress.

I can't say how disappointed I am. Has anyone experienced this or have any permanent solutions that can keep me using GeneratePress without suffering from this?