strange behavior with USB when MKR supplied via VIN with 5.0V

Hardware MKR Family MKRWIFI1010
#1

If I supply the MKR1010 with 5.0V via VIN, the USB interface no longer works. VIN gone, and USB is recognized again immediately.
VIN now reduced to 4.9V, USB works.
Should that be so or do I understand something wrong?

#2