Preview app not working properly in macOS Mojave
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:


Quit Preview
Move ~/Library/Preferences/com.apple.Preview.plist to the Trash
Re-launch Preview
Re-check preferences &amp; ensure Open all files in one window or Open groups of files in the same window is selected
Attempt to reproduce your original issue