USB serial disconnected with external power source


my MKR 1010 Wifi board works well when connected to USB cable alone.

When I put it on a breadboard with an external power source connected to VIN pin, it works with 3.3V but USB port is disconnected as soon as I power it with 5V !

I tried with another MKR 1010 board, with same result.

The board specs says it can be powered with 5V on VIN pin, so what's the point ?