Or maybe closing FTP-tab should disconnect?
No that would not work.
When you connect to a FTP site that path is connected to a virtual device (0:\ , 1:\, 2\ and so on) that is inserted into the Virtual filesystem inside MC.
And ANY Explorer Panel ( or other extension that browse the MC Virtual filesystem) will be able to see this device.
No tab is owning a filesystem device. You can start FTP from one tab and then browse to it from another.