URLs getting truncated in QuarkXpress running on macOS Mojave
Until recently this was actually a known problem in QuarkXPress (and I may add still is for a lot of other software that deals with URLs). Basically, the issue is that the maximum character length of the URL field in the New/Edit
Hyperlink box is 250 characters and, as a result, longer links were truncated. This issue is unrelated to macOS.

However, this issue was resolved when QuarkXpress 2018 was released in May 2018. I believe the issue still exists in QXP 2017 and earlier and, if so, you’d have to upgrade to QXP 2018.

If in fact you’re using QXP 2018, your best bet would be to check for any updates (you can do this via the Quark Update preference pane in Apple > System Preferences). If you’re using QuarkXpress 2018 Pro (the version sold on the Mac App Store), then you’ll need to check via the App Store for any updates.

However, my understanding is that this issue was resolved with release 14.0 (which was the first public release of QXP 2018 in May). See QuarkXPress 2018 Known and Resolved Issues.

If the issue still persists you’d be best to contact Quark Technical Support and/or visit their online forums.