It should be a session lock rather than a logoff.
Check your screensaver settings – currently xfce4-screensaver is the default one. https://docs.xfce.org/apps/screensaver/start
Go to Settings Editor in Manjaro Settings Manager
Scroll to xfce4-power-manager
Select inactivity-on-ac and change value from 14 to 0
Select inactivity-sleep-mode-on-ac and change value from 1 to 0