Problem: I am using Ubuntu 20.04 with dual monitor. Siwthing workspace with Super+Page Up/Page Down only happens on primary monitor.
*It seems that Ubuntu 18.04 have same action.
Resolution: the action above is the defaut action of Ubuntu, you can change it to switing workspace on all monitor after executing command:
gsettings set org.gnome.mutter workspaces-only-on-primary false