An isolator only isolates the outgoing terminals. The fly lead is an incoming terminal.
If you turn off an isolator it only isolates the outgoing terminals. the incoming line terminal is certainly of course still at phase voltage, and if there is any circuitry the incoming neutral would be live too if parted. If it's not parted it would be close to installation earth voltage.
This example can repeat in other situations so do take care - imagine a Usb double socket or similar. If you switch the socket off and leave the line connected and part the neutrals somewhere, you'd expect to get a shock from the neutral.
All current carrying conductors are to be treated as live for this reason, including neutral.
Glad you're ok and thanks for the reminder of the risks.