Logged in users with site under maintenance

Good morning, I think it's not a Gridbox issue but a Joomla issue, but I'll try asking.
It happens that when I close the site for maintenance, I still see many logged in users in the control panel. Even if I kick them all out, after a while, I see other users logged in.
I can't figure out how this is possible. It's definitely not a Gridbox issue, but can you tell me who I can contact?
Thanks!

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