After upgrading macOS to Mojave, Preview app is not functioning the same. I am no longer able to delete, drag & drop or select several files and open in the same window.
Is there a fix for this or do I need to go back to Sierra?
I would make a brand new user account on this Mac and be sure it's reproducible. I would hate for you to have user permissions corruption and have this be fixed with a new user.
Also - be sure to try a couple PDF you get from printing a web page to a PDF or other "clean" PDF that come from pages or word and print to PDF as opposed to export to PDF.
Sometimes a class of older or crafted / complicated PDF can cause issues that take a couple months for Apple to fix. I have a couple Mojave PDF related bugs that still aren't fixed since they are rare/obsucure/took a while to convince Apple they were legit.
Ensure in Preview's preferences, that either Open all files in one window or Open groups of files in the same window is selected. If that looks OK and you're still experiencing the issue: