After the forum upgrade, it appears "Invisible Mode" option is currently missing.
Normally, the invisible mode setting can be found at 'My Account'-->'General Settings'-->'Login & Privacy'.
As the result, users can't hide from being shown at 'Currently Active Users'.
Normally, the invisible mode setting can be found at 'My Account'-->'General Settings'-->'Login & Privacy'.
As the result, users can't hide from being shown at 'Currently Active Users'.