I made this setup again, with an ISO1540.
I noticed when I turn off my Nano which is "blinking" two LEDs over PCF8574, ESP32 just waits. When I turn Nano on, ESP32 moves on. So this is ok.
But when I turn off ESP32, there are problems. Nano keeps working fine, but ESP32 when turned on goes into resets. This can be fixed to remove VCC from ISO1540, the side connected to the ESP32. When ESP32 starts, then connect VCC on the ISO1540 and everything is fine.
In a project, this should be done with some MOSFET or so, I think. To power the IS01540 side after the ESP32.